The Council of Nicaea set the date of Easter in 325 A.D. They determined it to be the Sunday following the paschal full moon, which is the first full moon on or after the spring equinox.
So, Easter is always the first Sunday after the first full moon that falls on or after March 21.
That means it can fall between March 22 to April 25th.
You can read more about the significance of the paschal full moon here.