In a congress held in 1923, the Orthodox churches adopted a modified Gregorian calendar and decided to set the date of Easter according to the astronomical full moon for the meridian of Jerusalem. However, these changes have not been universally implemented, and a variety of practices remain among the Orthodox churches. The rule is that Easter is the first Sunday after the first ecclesiastical full moon that occurs on or after March 21. The lunar cycles used by the ecclesiastical system are simple to program. The following algorithm will compute the date of Easter in the Gregorian calendar system.
All variables are integers and all remainders from division are dropped.
The algorithm takes the year, y, and yields the month, m, and day, d, of Easter.