Easter Sunday falls on a different Sunday every year. Using the 1928 Book of Common Prayer of the Episcopal Church, Easter falls on the first Sunday, between March 21st and April 19th, after the Paschal full Moon that occurs on or after the spring equinox. If the full moon lands on a Sunday, then Easter is the following Sunday
The date is fixed in accordance with an ancient ecclesiastical computation, and does not always correspond to the astronomical equinox. The Metonic cycle of 19 years is one in which the phases of the Moon repeat exactly. It is thus possible to have a 19-year cycle for the dates of full or new Moon. In the Julian calendar this 19-year cycle can be fairly easily translated into a date for Easter.
The date of the Paschal Full Moon, used to determine the date of Easter, is based on mathematical approximations following a 19-year cycle called the Metonic cycle.
March 21 is the Church’s date of the March equinox, regardless of the time zone, while the actual date of the equinox varies between March 19 and March 22, and the date depends on the time zone.
In today’s Gregorian calendar the calculation is complicated by the definition of which century years are leap years. These leap years mess up the simple Metonic cycle by altering the number of days in different periods of 19 years.
On page lii and liii of the 1928 BCP, you can find the calculation for determining when Easter Sunday is. It requires the following steps:
- Find the Golden number or “Prime”
- Find the Sunday (Dominical) Letter
- Determine Easter Sunday on Calendar with item # 1,2