Why Easter Moves: How the Date Is Actually Determined
Easter is the anchor for a dozen other observances — Mardi Gras, Ash Wednesday, Palm Sunday, Good Friday, Pentecost — yet it lands anywhere between March 22 and April 25. The rule behind it is one of history's great calendar puzzles.
The rule
Easter falls on the first Sunday after the first full moon on or after March 21 (the church's fixed date for the spring equinox). Three moving parts interact: the solar year, the lunar month, and the seven-day week — which is why the date jumps around so much.
The "ecclesiastical" moon
Crucially, the church does not use the astronomical full moon. It uses the ecclesiastical full moon — a tabulated approximation fixed in 1582 with the Gregorian calendar reform. The tables (Metonic cycle plus correction terms) keep the lunar calendar aligned within a day or two of the real moon over centuries. This is why Easter occasionally differs from what a naive astronomical computation would give.
Computing it
The standard method is the "Anonymous Gregorian algorithm" (also called the Meeus/Jones/Butcher algorithm) — about ten integer divisions and remainders that encode the tables. It produces, for example: April 5 in 2026, March 28 in 2027, April 16 in 2028. Our Easter countdown uses exactly this algorithm, so the dates are correct indefinitely, not looked up from a list.
Why Orthodox Easter differs
Most Orthodox churches apply the same rule to the Julian calendar, whose March 21 currently falls 13 days later than the Gregorian one, and use older lunar tables. The result is usually one to five weeks after Western Easter — though occasionally (as in 2025) the two coincide.
The dependent holidays
- Mardi Gras: 47 days before Easter
- Ash Wednesday: 46 days before
- Palm Sunday: 7 days before
- Good Friday: 2 days before
- Ascension: 39 days after; Pentecost: 49 days after
Shift Easter and the whole chain shifts with it — one reason spring schedules (school breaks, tourism seasons, even financial quarters in some countries) vary so much year to year.