The Julian calendar, a solar calendar comprising 365 days in each year with an additional leap day every fourth year without exception, continues to serve as a religious calendar in certain segments of the Eastern Orthodox Church, Oriental Orthodoxy, and among the Amazigh people, also known as the Berbers.
Proposed in 46 BC by Julius Caesar and named after him, the Julian calendar aimed to reform the earlier Roman calendar, which primarily followed a lunisolar system. It officially commenced on January 1st, 45 BC, by Caesar's decree. Adopted as the predominant calendar in the Roman Empire and subsequently across much of the Western world for over 1,600 years, it held sway until 1582, when Pope Gregory XIII introduced a revised calendar.
The Julian calendar encompasses two kinds of years: a standard year of 365 days and a leap year of 366 days. These years adhere to a straightforward cycle of three standard years followed by one leap year, averaging a year length of 365.25 days. This figure exceeds the actual solar year length, approximately 365.2422 days (a value subject to variation), causing the Julian calendar to gain one day every 129 years, or 3.1 days every 400 years.
Gregory's calendar reform aimed to rectify the Julian calendar's discrepancy against the solar year by adjusting the average calendar year length to 365.2425 days. Consequently, the Gregorian calendar only gains 0.1 day over a 400-year period. As a result, for events occurring between 1901 and 2099, the Julian calendar lags behind the Gregorian calendar by 13 days (for instance, January 1st in the Julian calendar corresponds to January 14th in the Gregorian). While most Catholic nations swiftly embraced the new calendar, Protestant nations gradually followed suit over the ensuing two centuries. The majority of Orthodox nations maintain the Julian calendar for religious purposes but have adopted the Gregorian calendar as their civil calendar since the early 20th century