Why do Western and Eastern Easter sometimes coincide and sometimes not?

Both rules look for the first Sunday after the paschal full moon, but they read "full moon" from different calendars (Gregorian vs. Julian) and different astronomical tables. The dates align some years (e.g. 2025) and diverge by up to five weeks others.

Is this also called Computus?

Yes — "computus" is the historical Latin term for the calculation. The Anonymous Gregorian algorithm here was published in Nature in 1876 as an essentially shadowing of the centuries-old method.

Easter Date Calculator

Time

Easter is the first Sunday after the first ecclesiastical full moon on or after 21 March. The Anonymous Gregorian algorithm and Meeus's Julian algorithm both implement the rule exactly; this tool runs them for any year from 100 to 9999 and lists the next N upcoming Easters. The Orthodox dates are returned in the equivalent civil (Gregorian-calendar) date.

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Starting yearHow manyCalendar

Easter Sundays (10)

2026Apr 5Sun
2027Mar 28Sun
2028Apr 16Sun
2029Apr 1Sun
2030Apr 21Sun
2031Apr 13Sun
2032Mar 28Sun
2033Apr 17Sun
2034Apr 9Sun
2035Mar 25Sun

Gregorian Easter falls between 22 March and 25 April; Orthodox between 4 April and 8 May (Gregorian calendar).

How to use

Type a starting year (defaults to this one).
Pick how many years to project.
Switch between Gregorian (Western) and Orthodox (Eastern) — Catholic / Protestant churches use the first, Eastern Orthodox the second.

Frequently asked questions

Why do Western and Eastern Easter sometimes coincide and sometimes not?: Both rules look for the first Sunday after the paschal full moon, but they read "full moon" from different calendars (Gregorian vs. Julian) and different astronomical tables. The dates align some years (e.g. 2025) and diverge by up to five weeks others.
Is this also called Computus?: Yes — "computus" is the historical Latin term for the calculation. The Anonymous Gregorian algorithm here was published in Nature in 1876 as an essentially shadowing of the centuries-old method.