Nearly all Orthodox churches follow the Julian calendar for their liturgical years. Since the Julian year is about 11.25 minutes longer than the solar year, by the 1500s it had fallen 10 days behind. In 1582, Pope Gregory XIII introduced the more accurate Gregorian calendar, which the Western Church has used since. The Julian calendar currently is 13 days behind the Gregorian year.

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In 1923, a number of Orthodox churches adopted the Revised (New) Julian calendar. This new version aligns with the Gregorian calendar from March 1600 through February 2800. Pascha (Orthodox Easter) serves as the basis for the dates of other movable feasts such as Pentecost.