Easter does not always fall on a particular date, but it does always occur on the first Sunday following the Paschal Full Moon. Easter can fall on any Sunday between March 22 and April 25.
The date of Easter Sunday depends on when the Jewish Passover ends. Because Jewish holidays are based on solar and lunar cycles, the date of Easter is reliant on this calendar as well, making it a movable holiday. Over time, the Western Christian Church has standardized the system to make the holiday's date easier to determine. Each year, the date on which Easter falls is now determined by the Ecclesiastical calendar. According to this calendar, the first Ecclesiastical Full Moon date after March 20 is the Paschal Full Moon, and Easter falls on this date.