Why Does Easter's Date Change Every Year? The Computus Explained

Why does Easter move?

Easter is a moveable feast: its date is set not by the civil calendar but by the moon. The rule — known as the computus — is that Easter falls on the first Sunday after the first full moon on or after 21 March. Because the lunar cycle and the solar year don't divide evenly, that Sunday lands anywhere between 22 March and 25 April, shifting each year.

This single rule is why so much of the spring calendar moves. Good Friday, Easter Monday, Ascension Day, Pentecost, Whit Monday and Carnival are all fixed relative to Easter — so when Easter moves, they all move with it.

The rule, precisely

The computus uses an ecclesiastical approximation of astronomy, not live observation:

• The spring equinox is fixed at 21 March for the purpose of the calculation, regardless of the true astronomical equinox (which can fall on 19–21 March).
• The full moon is the ecclesiastical full moon, taken from long-standing lunar tables, which can differ from the astronomical full moon by a day or two.
• Easter is then the first Sunday after that ecclesiastical full moon.

The rule was standardised by the First Council of Nicaea in AD 325 to give all churches a common date, replacing a patchwork of local practices. The 22 March–25 April window has held ever since.

Why Orthodox Easter is usually different

Most Eastern Orthodox churches apply the same computus rule but reckon it on the older Julian calendar rather than the Gregorian calendar used in the West. The Julian calendar now runs 13 days behind, and its equinox and lunar tables have drifted — so Orthodox Easter is usually one to five weeks later than Western Easter. The two coincide only occasionally.

The Western dates follow the Gregorian reform of 1582 (Pope Gregory XIII); the Orthodox dates follow the unreformed Julian reckoning.

The holidays that move with Easter

Because these feasts are defined by their distance from Easter Sunday, their civil dates change every year. The offsets are fixed:

This is why a country's public-holiday count stays constant while the actual dates — and the long weekends they create — shift from year to year.

Frequently asked questions

What is the earliest and latest Easter can fall?

Under the Western (Gregorian) computus, Easter Sunday can fall no earlier than 22 March and no later than 25 April. Both extremes are rare; most years cluster in late March and April.

Why is Easter later in the Orthodox church?

Eastern Orthodox churches calculate Easter on the Julian calendar, which now lags the Gregorian by 13 days and uses older lunar tables. The result is usually one to five weeks later than Western Easter; the two dates coincide only in some years (2025, 2028, 2031 and 2034 in the current decade).

Does Easter depend on the real full moon?

Not exactly. The computus uses an ecclesiastical full moon from fixed tables and a fixed 21 March equinox, so the church date can differ from the astronomical full moon by a day or two.

Why do Good Friday and Whit Monday change dates too?

They are defined by their distance from Easter — Good Friday is two days before, Whit Monday fifty days after — so they move whenever Easter moves.

Sources

• The date rule was set by the First Council of Nicaea (AD 325) and revised for the West in the Gregorian calendar reform of 1582.
• Western vs Orthodox dates in the table are computed from the Gregorian and Julian computus respectively.

Related: what is Easter and its history, public holidays by country, and how we source and verify holiday data.