How the date of Easter is calculated
By Vijay Jadhav
Easter is the only major holiday in the Western calendar whose date is set by the Moon. It can fall on any Sunday from March 22 to April 25, which is why it feels early one year and late the next.
The rule
Western churches celebrate Easter on the first Sunday after the first full moon on or after March 21. That date, March 21, stands in for the spring equinox. The full moon in the rule is not the astronomical one you see in the sky but an "ecclesiastical" full moon taken from fixed tables, so every church gets the same answer.
Working it out
- Find the first ecclesiastical full moon on or after March 21.
- Move to the next Sunday. If the full moon is itself a Sunday, go to the following Sunday.
- That Sunday is Easter.
Mathematicians turned these tables into a short formula, often called the computus. This site uses the version published by the astronomer Jean Meeus, which gives the Western Easter date for any year in the Gregorian calendar.
Dates that follow Easter
- Mardi Gras is 47 days before Easter.
- Ash Wednesday is 46 days before Easter.
- Palm Sunday is one week before Easter.
- Good Friday is two days before Easter.
Why Orthodox Easter is often different
Most Orthodox churches use the same idea but with the older Julian calendar, so their Easter usually falls one to five weeks later. In some years the two dates match.