I found this explanation:
Believe it or not, Easter was not celebrated by the church until the 2nd Century. It was nearly always followed by 50 days focusing on the resurrection. There were two ways the date of Easter was celebrated. In Asia Minor, Christians celebrated Easter on the Jewish Passover, (14th of Nisan in the Jewish calendar) no matter what day of the week. Others used the Sunday after Passover. In 325 A.D., the church declared that Easter was to â€œbe on the Sunday that followed the first full moon after the spring equinox.â€ That means Easter can be as early as March 22nd and as late as April 25th.
The date for Easter for the Orthodox churches, which still use the Julian calendar, still differs from Western churches, which use the Gregorian calendar. As a result, the Easter date is usually one to four weeks later than in the West.