Easter’s date changes every year because it’s tied to the cycles of the Moon and the spring equinox, not to a fixed calendar day like Christmas on 25 December. In Western Christianity, Easter is a “movable feast” that can fall on any Sunday between 22 March and 25 April.

The core rule (simple version)

Most Western churches (Catholic and most Protestant) use this rule:

  • Easter is the first Sunday after the first full moon after the spring equinox (fixed as 21 March for church calculations).
  • That full moon is called the Paschal full moon, and Easter must come after it, never before.
  • Because full moons happen on a lunar cycle (about 29.5 days), Easter “moves” across late March and April each year.

So the short answer to “why does Easter date change?” is: it follows a combined solar–lunar rule instead of a simple “same date every year” rule.

Where that rule came from

Early Christians wanted Easter to stay linked to the Jewish festival of Passover, which is set by a lunar calendar. The New Testament places Jesus’ crucifixion and resurrection around Passover, so early churches tried to respect that timing.

  • In 325 CE, the Council of Nicaea decided Easter should be celebrated everywhere on:
    “The first Sunday after the first full moon on or after the spring equinox.”
  • To keep calculations consistent, the Church treated the equinox as always being on 21 March, even though the real astronomical date can shift by a day or two.

That decision fixed the method , but because the Moon’s phases shift year to year on our solar calendar, the date keeps changing.

Why different churches get different dates

There is another twist: not all churches use the same calendar to apply that rule.

  • Western churches (Catholic, most Protestant) use the Gregorian calendar, introduced in 1582 to correct the older Julian calendar.
  • Many Eastern Orthodox churches still use the Julian calendar for calculating Easter.

Both traditions follow the same basic formula, but:

  • Their “21 March” equinox date falls on different days relative to the modern Gregorian year.
  • They may also handle the Paschal full moon and its tables differently.

Because the Julian calendar now runs about 13 days behind the Gregorian, Orthodox Easter often falls one or more weeks later than Western Easter, and only sometimes coincides.

What range of dates are possible?

Using the Western (Gregorian) method:

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

Everything else—Good Friday, Palm Sunday, Ash Wednesday—moves with Easter because they are defined as a certain number of days or weeks before that shifting Sunday.

TL;DR: The date of Easter changes because churches deliberately tied it to a mix of the spring equinox and the lunar cycle (first Sunday after the first full moon after 21 March), and different Christian traditions even use different calendars to apply that rule.

Information gathered from public forums or data available on the internet and portrayed here.