The simple standard definition of Easter is that it is the first Sunday after the full Moon that occurs on or after the spring equinox. If the full Moon falls on a Sunday then Easter is the next Sunday.

What determines when Easter is each year?

Easter always occurs on the first Sunday after the Paschal Full Moon (the first full moon that occurs after the vernal equinox, which signifies the beginning of spring in the northern hemisphere), according to The Old Farmer's Almanac.

Why does the date of Easter change every year?

The date of Easter Sunday depends on the spring equinox

Unlike Christmas, the date of Easter is subject to change, as its date is determined by a lunisolar calendar.