A Sunday chosen by a rule
Western Easter changes date because its rule combines a Sunday with a calculated full moon, rather than choosing one fixed day of a month. It can fall between 22 March and 25 April.
Think of a birthday fixed on 10 April: it can land on any weekday. Easter works the other way round. The weekday stays Sunday, while the date moves. The United Methodist Church explains this moving celebration.
Follow the calendar, not a telescope
The US Naval Observatory gives the precise Western rule: find the first ecclesiastical full moon on or after 21 March, then take the Sunday after it. Ecclesiastical means that the church calendar supplies the date.
That full moon comes from tables. It is not necessarily the same date as the astronomical full Moon you could observe in the sky. The rule also fixes its equinox at 21 March, even when the real astronomical equinox occurs on another date.
Imagine that the qualifying calendar full moon falls on a Thursday. Easter is the Sunday three days later. If it falls on a Sunday, Easter waits until the following Sunday. The word after does some important work.
Which Easter does this explain?
This is the Gregorian calculation used by most Western Christian churches. Many Eastern Christian churches use a different calendar calculation, so their Easter can fall on another civil date. One rule should not be presented as every tradition’s practice.
You have already met the difference between a calendar label and elapsed time in Britain’s eleven skipped dates. Here, the same care helps separate a calendar Moon from the Moon we actually see.