How this instrument works
Unlike Christmas, Easter isn't fixed to a calendar date. Western (Gregorian) Easter Sunday is defined as the first Sunday after the first ecclesiastical full moon occurring on or after March 21 — a rule rooted in the old lunisolar reckoning of Passover, which is why the date jumps around from year to year, always landing somewhere between March 22 and April 25 inclusive.
Working that out doesn't require tracking real lunar phases. This instrument uses the 'Anonymous Gregorian algorithm,' also called the Meeus/Jones/Butcher algorithm after the astronomers who republished it — a sequence of modular-arithmetic steps that reproduces the ecclesiastical full moon and the following Sunday exactly, using only integer division and remainders. It was first published anonymously in the journal Nature in 1876 and has been the standard closed-form method ever since.
The algorithm starts from a 'golden number,' the year's position in the Moon's 19-year Metonic cycle (year mod 19), then layers on century-based corrections that account for the Gregorian calendar's own leap-year adjustments, before converting the result into a specific month and day. It's exact for every year the Gregorian calendar has been in effect and needs no lookup table.
- Enter Year — any year from 1583 (the year after the Gregorian calendar's adoption) onward.
- Read Easter Sunday for the full date, or Month and Day separately if you need them as individual numbers.
- Remember this computes Western/Gregorian Easter; Eastern Orthodox churches generally use the Julian calendar's Paschal calculation and often land on a different date.
Worked example — Easter Sunday in 2026
Enter 2026 into Year. Running the algorithm: the golden number a = 2026 mod 19 = 12, and the chain of century corrections works out to h = 12, l = 2, and m = 0. Feeding those into the final step gives month = ⌊128/31⌋ = 4 and day = (128 mod 31) + 1 = 5.
Easter Sunday reads 2026-04-05, with Month reading 4 and Day reading 5 — Easter Sunday falls on April 5, 2026, comfortably inside the algorithm's possible range of March 22 to April 25.
Questions
Why does Easter's date change every year instead of being fixed?
Because it's defined relative to a lunar event, not a calendar date: the first Sunday after the first ecclesiastical full moon on or after March 21. Since full moons don't line up neatly with the 365-day solar calendar, the specific Sunday that satisfies the rule shifts from year to year, landing anywhere between March 22 and April 25.
What algorithm does this instrument use to compute the date?
The Anonymous Gregorian algorithm, also known as the Meeus/Jones/Butcher algorithm — a sequence of integer-division and remainder steps first published in the journal Nature in 1876 and reprinted in Jean Meeus's Astronomical Algorithms in 1991. It's exact for any Gregorian-calendar year with no exceptions and no lookup table required.
Does this match Orthodox (Eastern) Easter too?
No. This instrument computes Western/Gregorian Easter, used by the Catholic Church and most Protestant denominations. Eastern Orthodox churches generally calculate their Paschal date using the older Julian calendar, which can land Orthodox Easter on the same date as Western Easter, or up to several weeks later, depending on the year.
Why can't I enter a year before 1583?
Because the Gregorian calendar itself wasn't adopted until October 1582, and this algorithm is defined in terms of Gregorian leap-year rules. 1583 is the first full year after adoption, so it's the earliest year the algorithm is meaningfully valid for.
Can I look up Easter dates far in the future, like the year 3000?
Yes — the algorithm is proleptic and mechanical, so it will compute a date for any year up to 9999 using the same Gregorian leap-year rules projected forward. Whether the calendar will actually still work that way that far out is a separate question from what the algorithm returns today.
What's the earliest and latest Easter can possibly fall?
The earliest possible date is March 22 and the latest is April 25, though both extremes are rare — most years land somewhere in the first three weeks of April. The full 35-day window comes directly from how the ecclesiastical full moon and the 'following Sunday' rule interact across different years.