The Julian calendar was the calendar in predominant use in most of Europe until it was superseded by the Gregorian calendar commencing in 1582, although it continued to be used as the civil calendar in some countries into the 20th century. The Gregorian calendar has now replaced the Julian calendar as the civil calendar in all countries which formerly used it.
Most Christian denominations in the West and areas evangelized by Western churches have also replaced it with the Gregorian calendar as the basis for their liturgical calendars. However, most branches of the Eastern Orthodox Church still use the Julian calendar for calculating the dates of moveable feasts, including Easter (Pascha). 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.
It was itself a reform of the Roman calendar introduced by Julius Caesar in 46 BC (708 AUC) to commence in 45 BC (709 AUC). The calendar has a regular year of 365 days divided into 12 months, as listed in Table of months. A leap day is added to February every four years. The Julian year is, therefore, on average 365.25 days long.
The calendar year was intended to approximate the tropical (solar) year. Although Greek astronomers had known, at least since Hipparchus, that the tropical year was a few minutes shorter than 365.25 days, the calendar did not compensate for this difference. As a result, the calendar year gained about three days every four centuries compared to observed equinox times and the seasons. This discrepancy was corrected by the Gregorian reform, introduced in 1582.