Easter Date Calculator

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.

The word ecclesiastical is doing the work in that definition. The full moon in the rule is not an astronomical event but a date from a table, computed so the church calendar stays fixed and predictable, and it can differ from the real full moon by a day or two. The equinox is fixed at 21 March by the same convention rather than observed, so a year where the astronomical equinox falls on the 20th changes nothing. Easter is a calendar construction, and that is why it can be computed centuries ahead without an ephemeris.

The two dates diverge for two separate reasons, which is why the gap between them is not constant. Orthodox churches compute the date on the Julian calendar, which has drifted thirteen days from the Gregorian, and they also use an older lunar table. The results can coincide, sit a week apart, or stand more than a month apart depending on how the two moons line up in a given year.

Within those rules Easter can land anywhere from 22 March to 25 April, a window of thirty-five days, and the extremes are rare: the last 22 March was 1818 and the next is 2285. Everything that hangs off Easter moves with it, which is what makes it worth computing rather than remembering — Ash Wednesday, Ascension and Pentecost are all fixed offsets from this one date, as are public holidays in much of Europe.

The algorithms are exact arithmetic rather than approximations, so a date for the year 4000 is as reliable as one for next year, assuming the calendar rules themselves survive that long.