The Julian Calendar is a solar calendar that was introduced in the Roman Empire, in the year now referred to (in the Common Calendar) as -44, during the reign of Julius Caesar, for whom it and its fifth month are named. The calendar was slightly reformed by emperor August, for whom the sixth month is named. After this reform, which was complete by the year now referred to as year 8 C.E., this calendar was used continuously in many regions of Europe until 1582 C.E., though there was variation (over both time and region) in such things as the used epoch, the used day count within each month, and which day was the first day of the year. The following description reflects the modern choices for these things.
In this calendar, each year consists of 12 months with between 28 and 31 days per month, with new days starting at midnight. Each new day starts at midnight. The English names of these months and their lengths are listed in the following table.
Number Month Name Length 1 January 31 2 February 28 (29 in a leap year) 3 March 31 4 April 30 5 May 31 6 June 30 7 July 31 8 August 31 9 September 30 10 October 31 11 November 30 12 December 31
In this calendar, years have either 365 or 366 days. The longer years, called leap years, occur every fourth year, whenever the year count is evenly divisible by 4. For example, the years 1000 and 1004 were leap years. The extra day is inserted at the end of the month of February, as the 29th day of February. January 1 is now the beginning of the new year, though this was not always the case.
Many different epochs have been used in conjunction with this calendar, for example, the assumed year of the foundation of the city of Rome (-753 C.E., referred to as year 1 A.U.C. – ab urbe condita), or the beginning of the reign of emperor Diocletian (284 C.E.), or the approximate birth year of the Messiah of the Christian faith (1 C.E. = 1 A.D. – anno Domini)
For dating historical events, use of the final rules of the Julian calendar (with the Common Era) is extended into the past. This anachronistic calendar is referred to as the Julian Proleptic Calendar, or, if confusion is unlikely, as the Julian Calendar.