Why does the Easter date change every year?

Easter is called a moveable feast because it is not celebrated
on a fixed date every year. Easter falls somewhere between late
March and late April each year, following the cycle of the moon.
After several centuries of disagreement, all churches accepted that
Easter is the first Sunday after the Paschal Full Moon, which is
the first full moon on or after March 21 (the ecclesiastic “vernal
equinox”).

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