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The Julian calendar was selected 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 probably intended to reflect some tropical year, with a standard year of 365 days divided into 12 months and a "leap day" added every 4 years. The calendar remained in use into the 20th century in some places. However with this scheme too many leap years are added with respect to the astronomical seasons which 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.
Various systems of year numbering were used with the Julian calendar, starting with ab urbe condita (from the supposed founding of Rome) 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. Around 527 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 (offset by 753 years from ab urbe condita).
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, which resulting 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 older Roman calendar, those tasked with implementing the calendar -- 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. It is probable that he decided to skip leap days in the twelve year period -8 AD (9 BC) to 3 AD inclusive. Thus the historic sequence of leap years (years with a leap day) may have been 43 BC, 40 BC, 37 BC, 34 BC, 31 BC, 28 BC, 25 BC, 22 BC, 19 BC, 16 BC, 13 BC, 10 BC, AD4, AD8, AD12 etc., or if papyrological evidence from Roman Egypt can be trusted 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, AD4, AD8, AD12 etc.
Because of Julius' and Augustus' contribution to the calendar, the Romans eventually named months after each of them, renaming Quintilis [Fifth month, with March = month 1] and Sextilis [Sixth Month].
The original scheme for months in the Julian Calendar may have been 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:
Augustus is said to have changed this in 8 BC to:
giving us the irregular month lengths which we still use today. Most scholars doubt Sacrobosco 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.
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. Sweden adopted the new style calendar in 1753, but did also for a 12 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, Christmas and New Year 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.