movable feast 
noun
A religious feast day that does not occur on the same calendar date each year, in particular Easter Day and other Christian holy days whose dates are related to it.That is, the Moon defines the dates of all moveable feasts in the liturgical year, reckoned from Easter, which is based on the full moon after the spring, or vernal, equinox....
- Easter is a moveable feast, the date of which is determined by the phases of the moon.
- The cleverness of this is that the Christian calendar's diet of moveable feasts keeps us guessing each year but we can't remember why.