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The Julian calendar began in 45 BC (709 AUC ) as a reform of the Roman calendar by Julius Caesar . It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year (known at least since Hipparchus ). The Julian calendar has a regular year of 365 days divided into 12 months with a leap day added to February every four years. The Julian year is, therefore, on average 365.25 days long. The motivation for most calendars is to fix the number of days between return of the cycle of seasons (from spring equinox to the next spring equinox, for example), so that the calendar could be used as an aid to planting and other season-related activities. The cycle of seasons (tropical year) had been known since ancient times to be about 365 and 1/4 days long.

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The more modern Gregorian calendar eventually superseded the Julian calendar: the reason is that a tropical year (or solar year) is actually about 11 minutes shorter than 365.25 days. These extra 11 minutes per year in the Julian calendar caused it to gain about three days every four centuries, when compared to the observed equinox times and the seasons. In the Gregorian calendar system, first proposed in the 16th century, this problem was dealt with by dropping some calendar days, in order to realign the calendar and the equinox times. Subsequently, the Gregorian calendar drops three leap year days across every four centuries.

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The Julian calendar remained in use into the 20th century in some countries as a civil calendar , but has been replaced by the Gregorian calendar in nearly all countries. The Roman Catholic Church , the Eastern Rite Catholic Churches and Protestant churches have replaced the Julian calendar with the Gregorian calendar; however, the Orthodox Church (with the exception of Romania , Estonia and Finland ) still use the Julian calendar for calculating the dates of moveable feasts . Some Orthodox churches have adopted the Revised Julian calendar for the observance of fixed feasts , while other Orthodox churches retain the Julian calendar for all purposes. The Julian calendar is still used by the Berber people of North Africa , and on Mount Athos .

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The Julian months were formed by adding ten days to a regular pre-Julian Roman year of 355 days, creating a regular Julian year of 365 days: Two extra days were added to Ianuarius , Sextilis (later changed in name to Augustus) and December, and one extra day was added to Aprilis , Iunius , September and November. Februarius was not changed in ordinary years, and so continued to be the traditional 28 days. Thus, the ordinary (i.e., non leap year) lengths of all of the months were set by the Julian calendar to the same values they still hold today. (See Debunked month length myths below for stories purporting otherwise).

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Immediately after the Julian reform, the twelve months of the Roman calendar were named Ianuarius , [3] Februarius , Martius , Aprilis , Maius , Iunius , Quintilis , Sextilis , September, October, November, and December, just as they were before the reform. The old intercalary month, the Mensis Intercalaris , was abolished and replaced with a single intercalary day at the same point (i.e. five days before the end of Februarius ). The first month of the year continued to be Ianuarius , as it had been since 153 BC.

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The Romans later renamed months after Julius Caesar and Augustus , renaming Quintilis (originally, "the fifth month", with March = month 1) as Iulius (July) [3] in 44 BC and Sextilis ("sixth month") as Augustus (August) in 8 BC. Quintilis was renamed to honour Caesar because it was the month of his birth. According to a senatus consultum quoted by Macrobius , Sextilis was renamed to honour Augustus because several of the most significant events in his rise to power, culminating in the fall of Alexandria, occurred in that month.

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Other months were renamed by other emperors, but apparently none of the later changes survived their deaths. Caligula renamed September ("seventh month") as Germanicus ; Nero renamed Aprilis (April) as Neroneus , Maius (May) as Claudius and Iunius (June) as Germanicus ; and Domitian renamed September as Germanicus and October ("eighth month") as Domitianus . At other times, September was also renamed as Antoninus and Tacitus , and November ("ninth month") was renamed as Faustina and Romanus . 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 .

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The Julian calendar was in general use in Europe and Northern Africa from the times of the Roman Empire until 1582, when Pope Gregory XIII promulgated the Gregorian calendar . Reform was required because too many leap days are added with respect to the astronomical seasons on the Julian scheme. On average, the astronomical solstices and the equinoxes advance by about 11 minutes per year against the Julian year. As a result, the calculated date of Easter gradually moved out of phase with the March equinox . While Hipparchus and presumably Sosigenes were aware of the discrepancy, although not of its correct value, it was evidently felt to be of little importance at the time of the Julian reform. However, it accumulated significantly over time: the Julian calendar gained a day about every 134 years. By 1582, it was ten days out of alignment from where it supposedly was in 325 during the Council of Nicaea .

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Months Lengths before 45 BC Lengths as of 45 BC Ianuarius 29 31 Februarius 28 (leap years: 23 or 24) 28 (leap years: 29) Martius 31 31 Aprilis 29 30 Maius 31 31 Iunius 29 30 Quintilis ( Iulius ) 31 31 Sextilis ( Augustus ) 29 31 September 29 30 October 31 31 November 29 30 December 29 31 Mercedonius/Intercalaris 0 (leap years: 27) (inserted between the shortened February and March) (abolished)

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In 45 BC, the Roman general Julius Caesar reformed the old Roman lunar calendar. The new calendar became known as the Julian calendar. It had 12 months and was based on the sun. a year lasted 365 days.every fourth year, a day was added, making the year 366 days long. These longer years are called leap years. This is because the extra day causes all the days following it to “leap”