Easter changes dates because it is based on the first Sunday after the first full moon following the spring equinox , not on a fixed day of the calendar. That is why it usually falls between March 22 and April 25 in Western churches.

Why it moves

  • The Church sets the spring equinox as March 21 for this calculation, even though the astronomical equinox can vary slightly.
  • The holiday then depends on the lunar cycle, so the date shifts from year to year.
  • This is also why Easter can sometimes line up with Passover timing, since both are tied to lunar calendars in different ways.

Why different churches differ

  • Most Western Christians use the Gregorian calendar, which is the civil calendar used in most of the world.
  • Eastern Orthodox churches usually use the Julian calendar for Easter calculations, so their Easter often lands on a different Sunday.
  • Because of that calendar gap, Orthodox Easter can be later than Western Easter, though they sometimes coincide.

Simple example

If the first full moon after March 21 happens on a Saturday, Easter is the next day. If it happens on a Sunday, Easter moves to the following Sunday.

Date range

  • Earliest possible Western Easter: March 22.
  • Latest possible Western Easter: April 25.

If you want, I can also show the rule in a one-line formula.