CALENDAR - The calendar in use at the time when Christianity began was the Julian, devised by Julius Caesar. The Russian Orthodox Church follows this calendar to this day in keeping the feasts and saints' days and fasts. The new calendar (Gregorian) was introduced in 1582 by Pope Gregory XIII and is used in the Western Churches and by some of the Eastern Churches.
The ecclesiastical calendar begins September 1. The liturgical calendar begins with Easter depending on the date when Easter is celebrated. The date of Easter was established at the First Ecumenical Council in 325.