The Eastern Apostolic Church follows the Old Calendar (Julian Calendar) for the Byzantine Rite and the Old Roman Calendar for the Latin Rite.
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The Julian Calendar was introduced in 46 BC by Julius Caesar and took effect in 45 BC (DCCIX ab urbe condita). It was developed in consultation with the Alexandrian astronomer Sosigenes [pronounced: Soh-SEE-ghe-ness] and was likely designed to approximate the tropical year. It has a regular year of 365 days, divided into 12 months, with a leap day added to February every four years. Hence, the Julian Calendar year is on average 365¼ days long.
The Julian Calendar remained in use into the 20th century in some countries, particularly in eastern Europe, and is still applied by the majority of Orthodox faithful for ecclesiastical dates.
One difficulty with the Julian Calendar is that too many leap days are added with respect to the astronomical seasons. On average, the astronomical solstices and the equinoxes advance by about 11 minutes per year against the Julian Calendar year, causing the Julian Calendar to drift back one day every 100 years (not divisible by 400). While Sosigenes was presumably aware of this discrepancy, although not of its correct value, it was evidently felt to be of little importance. However, the discrepancy accumulated significantly over time, and eventually led to the Calendar reform by Pope Gregory XIII in 1582 AD, which introduced the more astronomically accurate Gregorian Calendar. The notation Old Style (OS) is usually used to indicate a date in the Julian Calendar, as opposed to New Style (NS), which determines the date in the Gregorian Calendar. This notation is used when there would otherwise be confusion about which date is found in a given text.
While some Orthodox Christians celebrate the New Year on January 1st in the Gregorian Calendar, which predates the more widely used Old Calendar Calendar by 13 days. Thus, New Year falls on January 14th in the Old Calendar.
Most Eastern Orthodox jurisdictions base their liturgical calendar on the Julian Calendar, but some use a revised form of the Julian calendar. This was introduced in 1923 to bridge the gap between the Julian Calendar (used by the Eastern Orthodox Church) and the Gregorian Calendar used by the Western Churches (Roman Catholic, Old Catholic, Anglican, Protestant, etc.)