→From Julian to Gregorian: adding internal link
The Julian calendar was in general use in western Europe from the times of the Roman Empire until 1582, when Pope Gregory XIII promulgated the [[Gregorian Calendar]], which was soon adopted by most Roman Catholic countries. The Protestant countries followed later, and the countries of eastern Europe even later. Russia remained on the Julian Calendar until after the Russian Revolution (which is thus called the ''October Revolution'', although it occurred in November according to the Gregorian Calendar), in 1917, while Greece continued to use it until 1923.
Although all eastern European countries had adopted the Gregorian Calendar by 1923, the Orthodox Churches located in them had not. A [[Revised Julian Calendar]] was proposed during a synod in Constantinople in May of 1923, consisting of a solar part which was and will be identical to the Gregorian calendar until the year 2800, and a lunar part that calculated [[Pascha]] astronomically at Jerusalem. No local Orthodox Church accepted the lunar part of this proposal, so almost all Orthodox Churches continue to celebrate Pascha according to the Julian Calendar (the [[Church of Finland]] uses the Gregorian [[Paschalion]]). The solar part has only been accepted by certain Orthodox Churches, those of [[Church of Constantinople|Constantinople]], [[Church of Alexandria|Alexandria]], [[Church of Antioch|Antioch]], [[Church of Greece|Greece]], [[Church of Cyprus|Cyprus]], [[Church of Romania|Romania]], [[Church of Poland|Poland]], [[Church of Bulgaria|Bulgaria]], and the [[Orthodox Church in America]] (although the OCA's Diocese of Alaska and some other parishes retain the Julian Calendar). Thus, these churches celebrate the Nativity of Christ on the same day that Western Christians do because [[December 25]] coincides on both the Gregorian and Revised Julian Calendars (until 2800 when the Revised Julian Calendar will drop one day behind the Gregorian Calendar due to differing leap year rules).
The [[Old Calendarists]] and the Orthodox Churches of [[Church of Jerusalem|Jerusalem]], [[Church of Russia|Russia]], [[Church of Serbia|Serbia]], [[Church of Georgia|Georgia]], and [[Church of Ukraine|Ukraine]] continue to use the Julian Calendar for their fixed dates, thus they celebrate Nativity on [[December 25]] on the Julian Calendar, which corresponds to [[January 7]] on the Gregorian Calendar (until 2100, when the Julian Calendar will drift back one additional day with respect to the Gregorian Calendar).