Why Easter is on a different date every year

Christmas sits still; Easter wanders across five weeks of spring, dragging half the calendar with it. The reason is a rule fixed in 325 AD, an idealised Moon that exists only on paper, and a piece of arithmetic — the computus — that was, for centuries, the single most important calculation in Europe. This guide unpacks it; the Easter calculator runs the real algorithm for any year and shows the moveable feasts that follow.

The rule, stated properly

The rule everyone half-remembers: Easter is the first Sunday after the first full moon on or after March 21. Every clause earns its place. "On or after March 21" anchors it to (a stand-in for) the spring equinox. "First full moon" makes it lunar. "First Sunday after" makes it land on the Christian week's holy day — and the after is strict: if the full moon itself falls on a Sunday, Easter waits for the next one. The Council of Nicaea fixed this in 325 AD, largely so the whole church would keep the feast on one day instead of arguing regionally, which they had been doing enthusiastically.

The paper moon: why it's not astronomy

Here is the twist that surprises people: the "full moon" in the rule is not the one in the sky. The church needed every parish, from Ireland to Alexandria, to compute the same date without telescopes or time zones, so it defined an idealised ecclesiastical moon — a table-driven clockwork Moon that tracks the real one closely but not exactly. Likewise "March 21" is the church's fixed equinox, though the astronomical equinox can fall on the 19th or 20th. Occasionally the paper moon and the real sky disagree, and Easter lands a week from where an astronomer would put it. This is not a bug; it is the whole design — a shared computation beats a shared observation when the observers are a continent apart.

The computus: a 19-year clockwork

The machinery runs on the Metonic cycle: 19 solar years very nearly equal 235 lunar months, so the Moon's phases repeat against the calendar every 19 years. Each year's position in that cycle is its golden number (year mod 19, plus one), and from it the tables derive the paper moon's fullness in spring. The Gregorian reform of 1582 added correction gears — the leap-century adjustments in Leap years, explained properly and slow lunar corrections — because both the solar year and the Metonic approximation drift over centuries. The modern algorithm (the "anonymous Gregorian computus", published 1876) compresses all of it into a dozen integer divisions; it looks like arcane numerology and is in fact just those gear ratios written as arithmetic. It is the algorithm running inside the calculator, exact for every Gregorian year.

March 22 to April 25: the edge cases

The rule's earliest possible outcome: full moon on Saturday March 21, Easter the next day, March 22. It last happened in 1818 and will not happen again until 2285 — four and a half centuries between visits. The latest, April 25, requires the full moon to arrive on March 20 (a day too early), forcing a wait for the next lunation, which must then dodge a Sunday; it last landed in 1943 and returns in 2038, close enough that this article's readers will see it. The distribution between the extremes is lumpy — April 19 is the single most common date over the long run. Meanwhile mid-spring dates like April 5 (Easter 2026) feel "normal" precisely because the extremes are so rare.

Why Orthodox Easter differs

Eastern Orthodox churches apply the same Nicene rule but compute it on the Julian calendar, which now runs 13 days behind the Gregorian, with its own older lunar tables. The result: Orthodox Easter usually falls one to five weeks after Western Easter, occasionally coinciding (as in 2025). Same rule, different clockwork — a neat demonstration that the date was always a computation, not an observation. The wider story of why calendars drift and get patched is the leap-year story; the two articles are really one subject wearing two hats.

The five-week caravan that moves with it

Fix Easter and a whole season locks into place around it: Shrove Tuesday (Pancake Day) 47 days before, Ash Wednesday 46 before, Palm Sunday 7 before, Good Friday 2 before, Ascension 39 after, Pentecost 49 after. Schools set half-terms by it; economies notice it; and everyone privately re-asks the question this article answers each February. The calculator lists the next eight years with all the attendant feasts, and a countdown will happily tick toward the next one. Days-since handles the opposite direction, for anniversaries that don't move.