Easter changes dates because it’s tied to the Moon and the start of spring, not to a fixed calendar day like Christmas.

Quick Scoop

Easter is what’s called a “movable feast,” which means its date shifts every year instead of staying the same like 25 December. In most Western churches, Easter Sunday is defined as the first Sunday after the first full moon that comes after the spring (March) equinox.

The basic rule (in plain language)

  • The Church treats the spring equinox as always being 21 March.
  • Look for the first “Paschal Full Moon” (a church-defined full moon) on or after 21 March.
  • Easter is the next Sunday after that full moon.

Because the full moon can fall on many different dates each year, the Sunday that follows it also jumps around. That’s why Easter can land any time roughly between 22 March and 25 April.

How we ended up with this formula

Early Christians wanted Easter connected to Passover, which itself follows a lunisolar (moon + sun) calendar, not our modern purely solar-style calendar. In 325 CE, the Council of Nicaea tried to standardize things and settled on a rule based on the spring equinox and full moon rather than the strictly Jewish calendar dates.

Later, when Europe switched from the older Julian calendar to the more accurate Gregorian calendar, the Church kept the same basic rule but adjusted how dates and leap years were counted so Easter would stay in the spring season. That kept the tradition (moon + equinox) but aligned it better with the actual solar year.

Why different churches sometimes have different Easters

To make things even more “fun,” not all Christians calculate it the same way today.

  • Western churches (Catholic, many Protestants) use the Gregorian calendar and the equinox fixed at 21 March.
  • Many Eastern Orthodox churches use a formula tied to the older Julian calendar, which currently runs about 13 days behind the Gregorian one.

Because they’re using different base calendars and slightly different rules, Orthodox Easter can fall on a different Sunday , sometimes weeks apart from Western Easter.

Think of it like this

If Easter were just “the first Sunday in April,” it would be easy but would lose its ancient connection to both the spring season and the full moon that’s rooted in early Christian and Jewish tradition. By keeping this older pattern, we get a holiday whose date shifts with the sky rather than staying glued to a single box on the modern calendar.

TL;DR: Easter changes dates because it’s set as the first Sunday after the first full moon following the (church-defined) spring equinox on 21 March, and the Moon doesn’t follow our fixed calendar.