Each year, Easter falls on the first Sunday following the first ecclesiastical full moon occurring on or after March 21 (the established date of the vernal equinox). Easter always falls between March 22 and April 25.
Easter is the quintessential movable feast, a religious holiday that does not fall on the same calendar date each year. The method for determining when Easter is celebrated dates back to the days of Emperor Constantine of Rome in the fourth century A.D., as reported by the U.S. Naval Observatory. This original method was based on tables formed simply for the purpose of determining the date for celebrating Easter. The tables were fine tuned over the centuries, culminating with today's version, created during the formation of Pope Gregory XIII's revision of the Julian calendar. Eastern Christian churches never did adopt the Gregorian calendar. Therefore, those in the Eastern Orthodox Church use a slightly different method and sometimes end up celebrating Easter on a different date from those in western Catholic and Protestant churches.
Eastern churches determine Easter's date using a different point of reference. Instead of basing their calculation on the ecclesiastical full moon, they base their date for Easter on the astronomical full moon for the meridian of Jerusalem.