The Julian calendar was introduced by Julius Caesar in 46 BC, taking force in 45 BC or 709 ab urbe condita. It was chosen after consultation with Sosigenes and was obviously designed to approximate the tropical year as it was known at the time. It has a regular year of 365 days divided into 12 months, and a leap day is added every 4 years. The calendar remained in use into the 20th century in some places. However with this scheme too many leap days are added with respect to the astronomical seasons, which on average occur earlier in the calendar by about 11min per year. It is said that Caesar was aware of the discrepancy, but felt it was of little importance. In the 16th century the Gregorian Calendar Reform was introduced to improve its accuracy with respect to the time of vernal equinox, but the changes are relatively minor. Sometimes the reference Old Style or O.S., as opposed to 'New Style' for the Gregorian Calendar, is used when there is a confusion about which date is found in a text.
The previous Roman calendar contained various rules including two different lengths for leap month, plus modifications to the length of February during some leap years. This was further complicated by politics in which the calendar was modified to lengthen ones term in office. This resulted in the calendar becoming 90 days from its original intended definition. In order to re-align the calendar to what the Romans thought of as the correct seasons, 90 days were inserted. Because of its unusual length, the extra-long year was, and is, referred to as the Year of Confusion. The first year of operation of the new calendar was 45 BC, which was deemed to begin on January 1.
Despite the new calendar being much simpler than the Roman calendar, those tasked with implementing it -- the Pontiffs, not the Pope but a group of priests who were responsible for keeping the calendar in Roman society -- apparently misunderstood the algorithm. They added a leap day every 3 years. This resulted in too many leap days. Augustus Caesar remedied this discrepancy by skipping several leap days after 36 years of such mistakes.
The historic sequence of leap years (i.e. years with a leap day) in this period is not given explicitly by any ancient source, although the existence of the triennial cycle is confirmed by an inscription that dates from 9 or 8 BC. The chronologist Joseph Scaliger established in 1583 that the Augustan reform was instituted in 8 BC, and inferred that the sequence of leap years was 42 BC, 39 BC, 36 BC, 33 BC, 30 BC, 27 BC, 24 BC, 21 BC, 18 BC, 15 BC, 12 BC, 9 BC, AD 8, AD 12 etc. This proposal is still the most widely accepted solution.
Other solutions have been proposed from time to time. Kepler proposed in 1614 that the correct sequence of leap years was 43 BC, 40 BC, 37 BC, 34 BC, 31 BC, 28 BC, 25 BC, 22 BC, 19 BC, 16 BC, 13 BC, 10 BC, AD 8, AD 12 etc. In 1883 the German chronologist Matzat proposed 44 BC, 41 BC, 38 BC, 35 BC, 32 BC, 29 BC, 26 BC, 23 BC, 20 BC, 17 BC, 14 BC, 11 BC, AD 4, AD 8, AD 12 etc., based on a passage in Dio Cassius that mentions a leap day in 41 BC that was said to be contrary to Caesar's rule. It has also sometimes been suggested that 45 BC was a leap year. In the 1960s Radke argued the reform was actually instituted when Augustus became pontifex maximus in 12 BC, suggesting the sequence 45 BC, 42 BC, 39 BC, 36 BC, 33 BC, 30 BC, 27 BC, 24 BC, 21 BC, 18 BC, 15 BC, 12 BC, AD 4, AD 8, AD 12 etc.
In 1999, an Egyptian papyrus was published which gives an ephemeris table for 24 BC with both Roman and Egyptian dates. From this it can be shown that the most likely sequence was in fact 44 BC, 41 BC, 38 BC, 35 BC, 32 BC, 29 BC, 26 BC, 23 BC, 20 BC, 17 BC, 14 BC, 11 BC, 8 BC, AD 4, AD 8, AD12 etc., very close to that proposed by Matzat.
The Romans eventually named months after Caesar and Augustus, renaming Quintilis [Fifth month, with March = month 1] as Iulius (July) and Sextilis [Sixth Month] as Augustus (August). Other months were renamed from time to time (e.g. Septembris [Seventh Month] was renamed Germanicus) but these changes did not survive long.
According to the 13th century scholar Sacrobosco, the original scheme for months in the Julian Calendar was very regular, alternating long and short with an exception at the end of the year at the end of February. From January through December, the month lengths according to Sacrobosco were originally:
- 31, 29(30), 31, 30, 31, 30, 31, 30, 31, 30, 31, and 30
- 31, 28(29), 31, 30, 31, 30, 31, 31, 30, 31, 30, and 31
Sacrobosco is almost certainly wrong on this point. One thing that was not changed by the switch from the old Roman calendar to the new Julian calendar was the dates of the Nones and Ides. In particular, the Ides are late (on the 15th rather than 13th) in March, May, July and October. This suggests that these months always had 31 days in the Julian calendar. Also, Sacrobosco's theory is explicitly contradicted by the third and fifth century authors Censorinus and Macrobius, and, finally, it is inconsistent with seasonal lengths given by Varro, writing in 37 BC, before the Augustan reform, and with the 31-day Sextilis given by the new Egyptian papyrus from 24 BC.
Various systems of year numbering were used with the Julian calendar, starting with ab urbe condita (from the supposed founding of Rome, as calculated by Varro) or the reign year of the current ruler. Diocletian instituted anno Diocletiani, numbering from the beginning of his reign, which appears to have remained widespread after his death. In AD 525 Dionysius Exiguus proposed the system of anno Domini, which gradually spread through the Christian world. Years were numbered from the supposed date of the "incarnation" or "annunciation" of Christ on March 25 of the new year 1 (preceding the Diocletian era by 284 years, and offset by 753 years from ab urbe condita).
The Julian calendar was in general use in Europe from the times of the Roman Empire until 1582, when Pope Gregory XIII promulgated the Gregorian Calendar, which was soon adopted by most Catholic countries. The Protestant countries followed later, and the Eastern Orthodox ones yet later. England had Thursday 14 September 1752 follow Wednesday 2 September 1752. Sweden adopted the new style calendar in 1753, but also for a twelve-year period starting in 1700 use a modified Julian Calendar. Russia remained on the Julian calendar until after the Russian Revolution (which is thus called the 'October Revolution' but occurred in November according to the Gregorian calendar). The Eastern Orthodox churches themselves continued using the Julian Calendar until 1923, when many adopted their own Revised Julian Calendar rather than the Gregorian one. Easter, Pentecost, and their associated holy days are still calculated according to the Julian calendar in the Eastern Orthodox churches, and some Eastern Orthodox churches continue to use the Julian Calendar for all their church calendar dates.