The Church Calendar consists of a series of cycles by which feasts are celebrated in the Orthodox Church. There are currently three calendars in use in the Church, the Julian Calendar ("Old Calendar"), Revised Julian Calendar ("New Calendar"), and Gregorian Calendar ("New Calendar").
- The Julian Calendar churches are: Jerusalem, Russia, Serbia, Georgia, Poland, Sinai, Ukraine, and Japan.
- The Revised Julian Calendar churches are: Constantinople, Alexandria, Antioch, Romania, Bulgaria, Cyprus, Greece, Albania, Czech Lands and Slovakia, and the OCA.
A number of the above churches also have some parishes and dioceses which are on a different calendar than their respective primates, most especially the Ecumenical Patriarchate of Constantinople in the diaspora, which has many Julian Calendar parishes.
In terms of population, most Orthodox Christians follow the Julian Calendar (roughly 144 million to 41 million) but in terms of numbers of autocephalous and autonomous church bodies, the majority use one of the New Calendars (12 to 8).