Alex also provided the following algorithm that is based on the algorithm derived by the German mathematician Gauss, the principal simplification is that substitutions have been made for the case of Julian calendars and Orthodox Easters.This algorithm calculates the number of days AFTER March 21 (Julian) that Easter occurs (Note: It is a much simpler calculation than the Western Easter). R1=RMD(Year,19) R2=RMD(Year,4) R3=RMD(Year,7) RA=19*R1 16 R4=RMD(RA,30) RB=2*R2 4*R3 6*R4 R5=RMD(RB,7) RC=R4 R5 The number RC ranges from 1 to 35 which corresponds to March 22 to April 25 in the Julian Calendar (currently April 4 to May 8 on the Gregorian).
It is important to understand which method, which calendar and which year range applies to each method.
The Julian calendar applies to the original calculation method from 326 AD, which was the first year a unified Easter Dating method was used.
Except for sporadic use in the 1920's, the calendar reform was not adopted.
A wonderful resource examining the calendar reform and its lack of acceptance may be found in the excellent article .
Nevertheless, the Julian calendar is still used today by Orthodox churches as the basis for their Easter.