Easter has no fixed date because it is set by the moon as well as the sun. Western churches follow a rule that fits in one sentence: Easter is the first Sunday after the first full moon on or after March 21. Three details turn that rule into a date:
- March 21 is fixed. The church treats it as the spring equinox every year, whatever the astronomical equinox does.
- The full moon is calculated, not observed. The Paschal full moon comes from tables built on a 19-year lunar cycle, adopted with the Gregorian calendar in 1582, so it can be a day or two away from the full moon you see in the sky.
- Easter never falls on that full moon. When the Paschal full moon is on a Sunday, Easter moves to the Sunday after.
The result always lands between March 22 and April 25. Easter 2027, on March 28, is early in that window. After 2027, Easter next falls as late as April 25 in 2038, and as early as March 22 only in 2285.
Working out the date is called the computus. The MySavior countdown runs a standard version of it, the Meeus/Jones/Butcher algorithm, on your iPhone, so it works out Easter for every year without an app update. It counts 11 more of its feasts from Easter:
