How the Date of Easter Is Calculated

No holiday moves like Easter. Christmas owns a date; Easter owns a rule — and the rule is two thousand years old, slightly strange, and mathematically exact. Every year, a few divisions and remainders decide whether the egg hunt happens in March or late April. The machinery is called the computus, and it is one of the oldest algorithms still in daily use.

The rule that makes Easter move

The definition, settled in late antiquity and refined for the Gregorian calendar, reads like a riddle: Easter Sunday is the first Sunday after the ecclesiastical full moon that falls on or after March 21. Two cycles interlock — the weekly cycle of Sundays and the monthly cycle of moons — so the result wanders. Lunar months run about 29.5 days and never divide evenly into the solar year, so the qualifying "moon" slides later each year until the calendar resets it. The consequence is a 35-day window: Easter can land anywhere from March 22 to April 25, and in practice clusters in mid-April. The extremes are genuinely rare — March 22 last occurred in 1818 and will not return until 2285, while April 25 fell in 1943 and returns in 2038.

The ecclesiastical moon is not the real moon

The "full moon" in the rule deserves its qualifier. It is the ecclesiastical moon: a tabulated construction that approximates the real lunar cycle with a clean 19-year pattern, chosen so that any community with the table — not just trained astronomers — could agree on the date. The approximation is good but not perfect, and it can disagree with the sky by a day or two. That is why Easter occasionally falls just before an astronomically exact full moon without anyone considering it an error: the rule follows the table, and the table has been canonical since the sixth century.

Gauss's heirs: computus as integer arithmetic

Turning the rule into arithmetic for an arbitrary year occupied mathematicians for centuries — Gauss published a formula in 1800, with corrections — and the modern standard is a compact integer algorithm known as the Anonymous Gregorian algorithm (or Meeus/Jones/Butcher). For a year y it produces Easter's month and day from about a dozen divisions and remainders:

a = y mod 19            (position in the 19-year lunar cycle)
b = floor(y/100)  c = y mod 100
d = floor(b/4)    e = b mod 4
g = floor((b − f + 1)/3)   ... two century corrections
h = (19a + b − d − g + 15) mod 30   (ecclesiastical full moon)
l = (32 + 2e + 2i − h − k) mod 7    (day of the week shift)
m = floor((a + 11h + 22l)/451)
month = floor((h + l − 7m + 114)/31)
day   = ((h + l − 7m + 114) mod 31) + 1

No moon tables, no astronomy: pure number theory, exact for every Gregorian year. The Easter Date Calculator implements exactly this, which is why its output always agrees with the published calendars — try 2026 (April 5), 2027 (March 28) or the extreme year 2038 (April 25).

Why the Orthodox date is different

Eastern Orthodox churches celebrate Easter on the same astronomical intent but a different calendar. They compute on the Julian calendar, which today runs 13 days behind the Gregorian, and they use a stricter rule tying Easter to the Jewish Passover. Orthodox Easter therefore usually falls one to five weeks after the Western date, and the gap changes year to year — seven days in 2026, thirty-five in 2027. Occasionally the two systems land on the same Sunday, as in 2025, and the whole Christian world celebrates together. The Julian-to-Gregorian conversion the calculator applies grows by a day every non-leap century, which is also why its Orthodox results are warranted only through 4099.

The feasts that ride on Easter's coattails

Easter is the anchor of an entire family of movable observances, each at a fixed offset — so computing Easter computes them all. Ash Wednesday is 46 days before Easter, Palm Sunday 7 before, Good Friday 2 before, Easter Monday 1 after, Ascension 39 after and Pentecost 49 after. School holidays and civic calendars quietly inherit these dates, which is why the calculator lists them. For the general machinery of counting days toward any fixed target, see how a date countdown works.

Try it yourself

Open the Easter Date Calculator and check 2026 (Western April 5, Orthodox April 12), then 2038 to see the rare latest-possible Easter. Ask for 2025 and confirm both calendars coincide, then pull up any year's Ash Wednesday and Pentecost and notice they move in lockstep with Easter. Finally, enter a birth year and see which Easter week you were born into.