Why does Easter move every year?

The rule

Easter Sunday is the first Sunday after the first full moon on or after 21 March. The rule goes back to the Council of Nicaea in 325, which agreed that all Christians should celebrate Easter on the same Sunday; the tables that turn the rule into dates were worked out over the centuries that followed.

Two details make it a little different from what you would see in the sky. The "full moon" is not the real one but an ecclesiastical full moon, worked out from church tables that follow the real moon closely but not exactly. And the spring equinox is fixed at 21 March, even though the real equinox can be on the 19th, 20th or 21st.

How early and how late?

If the full moon falls on Saturday 21 March, Easter is the next day: 22 March, the earliest possible date. If a full moon has only just passed on 20 March, the next one is not until 18 April, and if that is a Sunday Easter waits another week — 25 April, the latest possible date.

Both extremes are rare. Easter was last on 22 March in 1818 and will not be again until 2285; it was last on 25 April in 1943 and will be next in 2038. Over the years 1583 to 4099, the range the standard Easter tables cover, the most common date is 10 April.

Why Orthodox Easter is often later

Most Orthodox churches use the same rule, but they apply it with the old Julian calendar, which is now 13 days behind the Gregorian calendar most of the world uses. Their 21 March is our 3 April, and their moon tables have drifted too. So the two Easters fall on the same Sunday only some years. Between 2000 and 2099:

• the same day: 31 years
• one week apart: 43 years
• four weeks apart: 4 years
• five weeks apart: 22 years

Easter in the next few years

Other days hang on Easter: Ash Wednesday is 46 days before it, Good Friday two days before, and Pentecost seven weeks after. Our Easter dates page lists them all for every year.