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The Julian calendar is a solar calendar of 365 days in every year with an additional leap day every fourth year (without exception). The Julian calendar is still used as a religious calendar in parts of the Eastern Orthodox Church and in parts of Oriental Orthodoxy as well as by the Amazigh (also known as the Berbers). For a quick calculation, between 1901 and 2099 the much more common Gregorian date equals the Julian date plus 13 days.
The Julian calendar was proposed in 46 BC by (and takes its name from) Julius Caesar, as a reform of the earlier Roman calendar, which was largely a lunisolar one. It took effect on , by his edict. Caesar's calendar became the predominant calendar in the Roman Empire and subsequently most of the Western world for more than 1,600 years, until 1582 when Pope Gregory XIII promulgated a revised calendar. Ancient Romans typically designated years by the names of ruling consuls; the Anno Domini system of numbering years was not devised until 525, and became widespread in Europe in the eighth century.
The Julian calendar has two types of years: a normal year of 365 days and a leap year of 366 days. They follow a simple cycle of three normal years and one leap year, giving an average year that is 365.25 days long. That is more than the actual solar year value of approximately 365.2422 days (the current value, which varies), which means the Julian calendar gains one day every 129 years. In other words, the Julian calendar gains 3.1 days every 400 years.
Gregory's calendar reform modified the Julian rule by eliminating occasional leap days, to reduce the average length of the calendar year from 365.25 days to 365.2425 days and thus almost eliminated the Julian calendar's drift against the Tropical year: the Gregorian calendar gains just 0.1 day over 400 years. For any given event during the years from 1901 through 2099, its date according to the Julian calendar is 13 days behind its corresponding Gregorian date (for instance Julian 1 January falls on Gregorian 14 January). Most Catholicism countries adopted the new calendar immediately; Protestantism countries did so slowly in the course of the following two centuries or so; most Orthodox countries retain the Julian calendar for religious purposes but adopted the Gregorian as their civil calendar in the early part of the twentieth century.
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If managed correctly this system could have allowed the Roman year to stay roughly aligned to a tropical year. However, since the Pontiff were often politicians, and because a Roman magistrate's term of official corresponded with a calendar year, this power was prone to abuse: a pontifex could lengthen a year in which he or one of his political allies was in office, or refuse to lengthen one in which his opponents were in power.Censorinus, De die natali 20.7 Latin)
Caesar's reform was intended to solve this problem permanently, by creating a calendar that remained aligned to the sun without any human intervention. This proved useful very soon after the new calendar came into effect. Varro used it in 37 BC to fix calendar dates for the start of the four seasons, which would have been impossible only 8 years earlier.Varro, On Agriculture I.1.28. A century later, when Pliny dated the winter solstice to 25 December because the sun entered the 8th degree of Capricorn on that date,Pliny, Natural History: (Book 18, LIX / LXVI / LXVIII / LXXIV). this stability had become an ordinary fact of life.
The octaeteris, a cycle of eight Lunar calendar popularised by Cleostratus (and also commonly attributed to Eudoxus) which was used in some early Greek calendars, notably in Attic calendar, is 1.53 days longer than eight mean Julian years. The length of nineteen years in the Metonic cycle was 6,940 days, six hours longer than the mean Julian year. The mean Julian year was the basis of the Callippic cycle devised by Callippus (a student under Eudoxus) to improve the Metonic cycle.
In Persia (Iran) after the reform in the Persian calendar by introduction of the Persian Zoroastrian (i. e. Young Avestan) calendar in 503 BC and afterwards, the first day of the year (1 Farvardin=Nowruz) slipped against the March equinox at the rate of approximately one day every four years.Hartner, Willy. "The young Avestan and Babylonian calendars and the antecedents of precession." Journal for the History of Astronomy 10 (1979): 1. pp. 1–22. Stern, Sacha. Calendars in antiquity: Empires, states, and societies. Oxford University Press, 2012., p. 178.
Likewise in the Egyptian calendar, a fixed year of 365 days was in use, drifting by one day against the sun in four years. An unsuccessful attempt to add an extra day every fourth year was made in 238 BC (Decree of Canopus). Caesar probably experienced this "wandering" or "vague" calendar in that country. He landed in the Nile delta in October 48 BC and soon became embroiled in the Ptolemaic dynastic war, especially after Cleopatra VII managed to be "introduced" to him in Alexandria.
Caesar imposed a peace, and a banquet was held to celebrate the event.Lucan, Pharsalia: Book 10. Lucan depicted Caesar talking to a wise man called Acoreus during the feast, stating his intention to create a calendar more perfect than that of Eudoxus (Eudoxus was popularly credited with having determined the length of the year to be days).Émile Biémont, Rythmes du temps, astronomie et calendriers, éd. De Boeck (Bruxelles), 2000 (), p. 224. But the war soon resumed and Caesar was attacked by the Egyptian army for several months until he achieved victory. He then enjoyed a long cruise on the Nile with Cleopatra before leaving the country in June 47 BC.Suetonius, Caesar 52.1.
Caesar returned to Rome in 46 BC and, according to Plutarch, called in the best philosophers and mathematicians of his time to solve the problem of the calendar.Plutarch, Lives of the Noble Grecians and Romans: Caesar 59. Pliny says that Caesar was aided in his reform by the astronomer Sosigenes of AlexandriaPliny, Natural History: (Book 18, LVII). who is generally considered the principal designer of the reform. Sosigenes may also have been the author of the astronomical almanac published by Caesar to facilitate the reform.Encyclopædia Britannica Sosigenes of Alexandria. Eventually, it was decided to establish a calendar that would be a combination between the old Roman months, the fixed length of the Egyptian calendar, and the days of Greek astronomy. According to Macrobius, Caesar was assisted in this by a certain Marcus Flavius.Macrobius, Saturnalia I.14.2 (Latin).
The three most important of these calendars are the Coptic calendar and the Ancient Macedonian calendar─which had two forms: the Syro-Macedonian and the Roman Asia calendars. Other reformed calendars are known from Cappadocia, Cyprus and the cities of (Roman) Syria and Palestine. Unreformed calendars continued to be used in Gaul (the Coligny calendar), Greece, Macedon, the Balkans and parts of Palestine, most notably in Judea.
The Asian calendar was an adaptation of the Ancient Macedonian calendar used in the Roman province of Asia and, with minor variations, in nearby cities and provinces. It is known in detail through the survival of decrees promulgating it issued in 8BC by the proconsul Paullus Fabius Maximus. It renamed the first month Dios as Kaisar, and arranged the months such that each month started on the ninth day before the kalends of the corresponding Roman month; thus the year began on 23 September, Augustus's birthday.
These months are called Intercalaris Prior and Intercalaris Posterior in letters of Cicero written at the time; there is no basis for the statement sometimes seen that they were called "Undecimber" and "Duodecember", terms that arose in the 18th century over a millennium after the Roman Empire's collapse.E.g., "... we have a sidelight on what was involved in "the year of confusion" as it was called. According to Dion Cassius, the historian, there was a governor in Gaul who insisted that, in the lengthened year, two months' extra taxes should be paid. The extra months were called Undecimber and Duodecimber." (P. W. Wilson, The romance of the calendar (New York, 1937), 112). The eponymous dating of the cited passage ( Dio Cassius 54.21) shows that it actually refers to an event of 15 BC, not 46 BC. Their individual lengths are unknown, as is the position of the Nones and Ides within them.J. Rüpke, The Roman Calendar from Numa to Constantine: Time, History and the Fasti, 117f., suggests, based on the ritual structures of the calendar, that 5 days were added to November and that the two intercalary months each had 31 days, with Nones and Ides on the 7th and 15th.
Because 46 BC was the last of a series of irregular years, this extra-long year was, and is, referred to as the "last year of confusion". The new calendar began operation after the realignment had been completed, in 45 BC.William Smith, Dictionary of Greek and Roman Antiquities: Year of Julius Caesar, following Ideler, interprets Macrobius, Saturnalia 1.14.13 (Latin) to mean that Caesar decreed that the first day of the new calendar began with the new moon which fell on the night of 1/2 January 45 BC.
The new moon was on 2 January 45 BC (in the Proleptic Julian calendar) at 00:21 UTC, according to IMCCE (a branch of the Paris Observatory): Phases of the moon (between −4000 and +2500) . This is in agreement with the historical moon phase tables by Fred Espenak in which the new moon was on 2 January 45 BC at 00:43 UTC. Espenek's table assumes that the first Julian year of 45 BC was a leap year. If the first year of 45 BC was not a leap year, there would be a day offset, and the new moon would have been on 1 January 45 BC at 00:43 UTC.
Espnek's historical moon phase tables also indicate that there was a new moon on 1 March 45 BC at 08:39 UTC (Calends of March), quarter moon on 8 March 45 BC at 09:00 UTC (a day after Nones of March), and full moon on 15 March 45 BC at 07:19 UTC (Ides of March). Espenak's tables of the phases of the moon are based on computational procedures described in Astronomical Algorithms by Jean Meeus (Willmann-Bell, Inc., Richmond, 1998).
More recent studies of the Macrobius manuscripts have shown that the word on which Idler's supposition is based, which was read as lunam, should be read as linam, meaning that Macrobius was simply stating that Caesar published an edict giving the revised calendar – see e.g., p.99 in the translation of Macrobius by P. Davies.
Smith gives no source or justification for his other speculation that Caesar originally intended to commence the year precisely with the winter solstice.
The Julian reform did not change the method used to account days of the month in the pre-Julian calendar, based on the Kalends, Nones and Ides, nor did it change the positions of these three dates within the months. Macrobius states that the extra days were added immediately before the last day of each month to avoid disturbing the position of the established religious ceremonies relative to the Nones and Ides of the month.Macrobius, Saturnalia 1.14.9 (Latin). Exceptionally, the extra day in April was inserted as the 26th, a.d. VI Kal. Mai. in the Julian calendar, in order to avoid adding a day to the Floralia, which ran from a.d. IV Kal. Mai. (27 April in the pre-Julian calendar) to a.d. V Non. Mai.
The inserted days were all initially characterised as dies fasti ( F – see Roman calendar).Macrobius, Saturnalia 1.14.12 (Latin). The character of a few festival days was changed. In the early Julio-Claudian period a large number of festivals were decreed to celebrate events of dynastic importance, which caused the character of the associated dates to be changed to NP. However, this practice was discontinued around the reign of Claudius, and the practice of characterising days fell into disuse around the end of the first century AD: the Antonine jurist Gaius speaks of dies nefasti as a thing of the past.A. K. Michels, The Calendar of the Roman Republic Appendix II; J. Rüpke, The Roman Calendar from Numa to Constantine 113–114, 126–132, 147.
There is debate about the exact position of the bissextile day in the early Julian calendar. The earliest direct evidence is a statement of the 2nd century jurist Celsus, who states that there were two-halves of a 48-hour day, and that the intercalated day was the "posterior" half. An inscription from AD 168 states that a.d. V Kal. Mart. was the day after the bissextile day. The 19th century chronologist Ideler argued that Celsus used the term "posterior" in a technical fashion to refer to the earlier of the two days, which requires the inscription to refer to the whole 48-hour day as the bissextile. Some later historians share this view. Others, following Theodor Mommsen, take the view that Celsus was using the ordinary Latin (and English) meaning of "posterior". A third view is that neither half of the 48-hour "bis sextum" was originally formally designated as intercalated, but that the need to do so arose as the concept of a 48-hour day became obsolete.W. Sternkopf, " Das Bissextum", (JCP 41 (1895) 718–733).
There is no doubt that the bissextile day eventually became the earlier of the two days for most purposes. In 238 Censorinus stated that it was inserted after the Terminalia (23 February) and was followed by the last five days of February, i.e., a.d. VI, V, IV, III and prid. Kal. Mart. (which would be 24 to 28 February in a common year and the 25th to 29th in a leap year). Hence he regarded the bissextum as the first half of the doubled day. All later writers, including Macrobius about 430, Bede in 725, and other medieval computus (calculators of Easter) followed this rule, as does the liturgical year of the Roman Catholic Church. However, Celsus' definition continued to be used for legal purposes. It was incorporated into Justinian's Digest,Justinian, Digest 50.16.98 . and in the English Statute De Anno et Die Bissextili of 1236, which was not formally repealed until 1879.
The effect of the bissextile day on the nundinal cycle is not discussed in the sources. According to Dio Cassius, a leap day was inserted in 41 BC to ensure that the first market day of 40 BC did not fall on 1 January, which implies that the old 8-day cycle was not immediately affected by the Julian reform. However, he also reports that in AD 44, and on some previous occasions, the market day was changed to avoid a conflict with a religious festival. This may indicate that a single nundinal letter was assigned to both halves of the 48-hour bissextile day by this time, so that the Regifugium and the market day might fall on the same date but on different days. In any case, the 8-day nundinal cycle began to be displaced by the 7-day Seven-day week in the first century AD, and began to appear alongside nundinal letters in the fasti. Dio Cassius 48.33.4, 60.24.7; C. J. Bennett, "The Imperial Nundinal Cycle", Zeitschrift für Papyrologie und Epigraphik 147 (2004) 175–179.
Although Greek astronomers had known, at least since Hipparchus,Ptolemy, tr. G. J. Toomer, Almagest, 1998, Princeton University Press, p. 139. Hipparchus stated that the "solar year ... contains 365 days, plus a fraction which is less than by about th of the sum of one day and night". a century before the Julian reform, that the tropical year was slightly shorter than 365.25 days, the calendar did not compensate for this difference. As a result, the calendar year gains about three days every four centuries compared to observed equinox times and the seasons. This discrepancy was largely corrected by the Gregorian reform of 1582. The Gregorian calendar has the same months and month lengths as the Julian calendar, but, in the Gregorian calendar, year numbers evenly divisible by 100 are not leap years, except that those evenly divisible by 400 remain leap years Introduction to Calendars . (15 May 2013). United States Naval Observatory. (even then, the Gregorian calendar diverges from astronomical observations by one day in 3,030 years).
MacrobiusMacrobius Ambrosius Theodosius, Saturnalia, 1.14.13–1.14.14, tr. Percival Vaughan Davies, New York 1969, Latin text at [29] gives the following account of the introduction of the Julian calendar:
So, according to Macrobius,
Some people have had different ideas as to how the leap years went. The above scheme is that of Scaliger (1583) in the table below. He established that the Augustan reform was instituted in 8 BC. The table below shows for each reconstruction the implied proleptic Julian date for the first day of Caesar's reformed calendar and the first Julian date on which the Roman calendar date matches the Julian calendar after the completion of Augustus' reform.
By the systems of Scaliger, Ideler and Bünting, the leap years prior to the suspension happen to be BC years that are divisible by 3, just as, after leap year resumption, they are the AD years divisible by 4.
Pierre Brind'AmourPierre Brind'Amour, Le calendrier romain, Ottawa 1983, pp. 45–46. argued that "only one day was intercalated between 1/1/45 and 1/1/40 (disregarding a momentary 'fiddling' in December of 41)Dio Cassius 48.33.4, tr. Earnest Cary, Loeb Classical Library, 9 vol., Harvard 1914–1927, available at [39]. to avoid the nundinum falling on Kal. Ian."Refutation of Brind'Amour's theory by John Ward, Re: Intercalation in 45BC to 8AD, East Carolina University Calendar discussion List CALNDR-L, April 1998.
Alexander Jones says that the correct Julian calendar was in use in Egypt in 24 BC, implying that the first day of the reform in both Egypt and Rome, , was the Julian date 1 January if 45 BC was a leap year and 2 January if it was not. This necessitates fourteen leap days up to and including AD 8 if 45 BC was a leap year and thirteen if it was not. In 1999, a papyrus was discovered which gives the dates of astronomical phenomena in 24 BC in both the Egyptian and Roman calendars. From , Egypt had two calendars: the old Egyptian in which every year had 365 days and the new Alexandrian in which every fourth year had 366 days. Up to the date in both calendars was the same. The dates in the Alexandrian and Julian calendars are in one-to-one correspondence except for the period from 29 August in the year preceding a Julian leap year to the following 24 February.Dieter Hagedorn, Zum aegyptischen Kalender unter Augustus, Zeitschrift fűr Papyrologie und Epigraphik 100 (1994) 211–222, available at [40]. From a comparison of the astronomical data with the Egyptian and Roman dates, Alexander JonesAlexander Jones, Calendrica II: Date Equations from the Reign of Augustus, Zeitschrift fűr Papyrologie und Epigraphik 129 (2000) 159–166, available at [41]. concluded that the Egyptian astronomers (as opposed to travellers from Rome) used the correct Julian calendar.
Due to the confusion about this period, we cannot be sure exactly what day (e.g. Julian day number) any particular Roman date refers to before March of 8 BC, except for those used in Egypt in 24BC which are secured by astronomy.
An inscription has been discovered which orders a new calendar to be used in the Province of Asia to replace the previous Greek lunar calendar. OGIS 458 (Greek). According to one translation
This is historically correct. It was decreed by the proconsul that the first day of the year in the new calendar shall be Augustus' birthday, a.d. IX Kal. Oct. Every month begins on the ninth day before the kalends. The date of introduction, the day after 14 Peritius, was 1 Dystrus, the next month. The month after that was Xanthicus. Thus Xanthicus began on a.d. IX Kal. Mart., and normally contained 31 days. In leap year, however, it contained an extra "Sebaste day", the Roman leap day, and thus had 32 days. From the lunar nature of the old calendar we can fix the starting date of the new one as 24 January, in the Julian calendar, which was a leap year. Thus from inception the dates of the reformed Asian calendar are in one-to-one correspondence with the Julian.
Another translation of this inscription is
This would move the starting date back three years to 8 BC, and from the lunar synchronism back to 26 January (Julian). But since the corresponding Roman date in the inscription is 24 January, this must be according to the incorrect calendar which in 8 BC Augustus had ordered to be corrected by the omission of leap days. As the authors of the previous paper point out, with the correct four-year cycle being used in Egypt and the three-year cycle abolished in Rome, it is unlikely that Augustus would have ordered the three-year cycle to be introduced in Asia.
Other months were renamed by other emperors, but apparently none of the later changes survived their deaths. In AD 37, Caligula renamed September as "Germanicus" after his Germanicus;Suetonius, Caligula 15.2. in AD 65, Nero renamed April as "Neroneus", May as "Claudius" and June as "Germanicus";Tacitus, Annals 15.74, 16.12. and in AD 84 Domitian renamed September as "Germanicus" and October as "Domitianus".Suetonius, Domitian 13.3. Commodus was unique in renaming all twelve months after his own adopted names (January to December): "Amazonius", "Invictus", "Felix", "Pius", "Lucius", "Aelius", "Aurelius", "Commodus", "Augustus", "Herculeus", "Romanus", and "Exsuperatorius". Dio Cassius 73.15.3. The emperor Tacitus is said to have ordered that September, the month of his birth and accession, be renamed after him, but the story is doubtful since he did not become emperor before November 275.Historia Augusta, Tacitus 13.6. On the chronology see R. McMahon, Tacitus. Similar honorific month names were implemented in many of the provincial calendars that were aligned to the Julian calendar.Surveyed in K. Scott, Honorific Months, Yale Classical Studies 2 (1931) 201–278.
Other name changes were proposed but were never implemented. Tiberius rejected a senatorial proposal to rename September as "Tiberius" and October as "Livius", after his mother Livia.Suetonius, Tiberius 26.2. Antoninus Pius rejected a senatorial decree renaming September as "Antoninus" and November as "Faustina", after his empress.Historia Augusta, Antoninus Pius 10.1.
In 309 and 310, and from time to time thereafter, no consuls were appointed. When this happened, the consular date was given a count of years since the last consul (called "post-consular" dating). After 541, only the reigning emperor held the consulate, typically for only one year in his reign, and so post-consular dating became the norm. Similar post-consular dates were also known in the west in the early 6th century. The system of consular dating, long obsolete, was formally abolished in the law code of Leo VI, issued in 888.
Only rarely did the Romans number the year from the founding of the city (of Rome), ab urbe condita (AUC). This method was used by Roman historians to determine the number of years from one event to another, not to date a year. Different historians had several different dates for the founding. The Fasti Capitolini, an inscription containing an official list of the consuls which was published by Augustus, used an epoch of 752 BC. The epoch used by Varro, 753 BC, has been adopted by modern historians. Indeed, Renaissance editors often added it to the manuscripts that they published, giving the false impression that the Romans numbered their years. Most modern historians tacitly assume that it began on the day the consuls took office, and ancient documents such as the Fasti Capitolini which use other AUC systems do so in the same way. However, Censorinus, writing in the 3rd century AD, states that, in his time, the AUC year began with the Parilia, celebrated on 21 April, which was regarded as the actual anniversary of the foundation of Rome. Because the lively festivities associated with the Parilia conflicted with the solemnity of Lent, which was observed until the Saturday before Easter Sunday, the early Roman church did not celebrate Easter after 21 April.
Many local eras, such as the Era of Actium and the Spanish Era, were adopted for the Julian calendar or its local equivalent in the provinces and cities of the Roman Empire. Some of these were used for a considerable time.For a partial survey see Samuel introduces his survey by saying: "The number of eras which came into use and then expired to be replaced by yet other eras during Hellenistic and Roman times is probably not infinite, but I have not been able to find the end of them." Anatolian eras are exhaustively surveyed in Perhaps the best known is the Era of Martyrs, sometimes also called Anno Diocletiani (after Diocletian), which was associated with the Alexandrian calendar and often used by the Christians to number their Easters during the 4th and 5th centuries, and continues to be used by the Coptic and Ethiopian churches.
In the eastern Mediterranean, the efforts of Christian chronographers such as Annianus of Alexandria to date the Biblical creation of the world led to the introduction of Anno Mundi eras based on this event. The most important of these was the Etos Kosmou, used throughout the Byzantine world from the 10th century and in Russia until 1700. In the west, the kingdoms succeeding the empire initially used indictions and , alone or in combination. The chronicler Prosper of Aquitaine, in the fifth century, used an era dated from the Passion of Christ, but this era was not widely adopted. Dionysius Exiguus proposed the system of Anno Domini in 525. This era gradually spread through the western Christian world, once the system was adopted by Bede in the eighth century.
The Julian calendar was also used in some Muslim countries. The Rumi calendar, the Julian calendar used in the later years of the Ottoman Empire, adopted an era derived from the lunar Islamic calendar year equivalent to AD 1840, i.e., the effective Rumi calendar was AD 585. In recent years, some users of the Berber calendar have adopted an era starting in 950 BC, the approximate date that the Libyan pharaoh came to power in Egypt.
In Anglo-Saxon England, the year most commonly began on 25 December, which, as (approximately) the winter solstice, had marked the start of the year in pagan times, though 25 March (the equinox) is occasionally documented in the 11th century. Sometimes the start of the year was reckoned as 24 September, the start of the so-called "western indiction" introduced by Bede.M. L. R. Beaven, "The Regnal Dates of Alfred, Edward the Elder, and Athelstan", English Historical Review 32 (1917) 517–531; idem, "The Beginning of the Year in the Alfredian Chronicle (866–87)", English Historical Review 33 (1918) 328–342. These practices changed after the Norman conquest. From 1087 to 1155 the English year began on 1 January, and from 1155 to 1751 it began on 25 March.Catholic Encyclopedia, General Chronology (Beginning of the Year). In 1752 it was moved back to 1 January. (See [Calendar Act 1750]]).
Even before 1752, 1 January was sometimes treated as the start of the new year – for example by Pepys Pepys Diary, "I sat down to end my journell for this year, ..." – while the "year starting 25th March was called the Civil or Legal Year".Spathaky, Mike Old Style and New Style dates and the change to the Gregorian calendar. To reduce misunderstandings on the date, it was not uncommon for a date between 1 January and 24 March to be written as "1661/62". This was to explain to the reader that the year was 1661 counting from March and 1662 counting from January as the start of the year.Spathaky, Mike Old Style and New Style dates and the change to the Gregorian calendar. "An oblique stroke is by far the most usual indicator, but sometimes the alternative final figures of the year are written above and below a horizontal line, as in a fraction (a form which cannot easily be reproduced here in ASCII text). Very occasionally a hyphen is used, as 173334." (For more detail, see Dual dating).
A calendar similar to the Julian one, the Alexandrian calendar, is the basis for the Ethiopian calendar, which is still the civil calendar of Ethiopia. Egypt converted from the Alexandrian calendar to Gregorian on 1 Thaut 1592/11 September 1875. Papers Relating to the Foreign Relations of the United States, vol. 2, pp. 1348–1349.
During the changeover between calendars and for some time afterwards, dual dating was used in documents and gave the date according to both systems. In contemporary as well as modern texts that describe events during the period of change, it is customary to clarify to which calendar a given date refers by using an O.S. or N.S. suffix (denoting Old Style, Julian or New Style, Gregorian).
Since the Julian and Gregorian calendars were long used simultaneously, although in different places, calendar dates in the transition period are often ambiguous, unless it is specified which calendar was being used. In some circumstances, double dates might be used, one in each calendar. The notation "Old Style" (O.S.) is sometimes used to indicate a date in the Julian calendar, as opposed to "New Style" (N.S.), which either represents the Gregorian date or the Julian date with the start of the year as 1 January. This notation is used to clarify dates from countries that continued to use the Julian calendar after the Gregorian reform, such as Great Britain, which did not adopt the reformed calendar until 1752, or Russia, which did not do so until 1918 (see Soviet calendar). This is why the Russian Revolution of 7 November 1917 N.S. is known as the October Revolution, because it began on 25 October O.S.
The Orthodox Churches of Jerusalem, Russia, Serbia, Montenegro, Poland (from 15 June 2014), North Macedonia, Georgia, and the Greek Old Calendarists and other groups continue to use the Julian calendar, thus they celebrate the Nativity on 25 December Julian (which is 7 January Gregorian until 2100).
The Orthodox Church of Ukraine announced in late May 2023 that they would use the Gregorian calendar to celebrate Christmas on December 25, 2023, partly in reflection to Russia's invasion of the country in early 2022; the church continues to celebrate Easter on the date according to the Julian tradition.
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