Why is it that Christmas has a fixed date, Dec. 25th, and Easter does not? If the first Christmas has a date that doesn't change, wouldn't it make sense to have a similar date for Easter?
I guess that since Easter (i) has to fall on a Sunday and (ii) has to fall around Passover, the early Church had to make it a movable feast. Since the Annunciation and Christmas do not need to fall on a particular day of the week and have nothing to do with the Jewish calendar, the early Church was able to designate March 25 and December 25 as set holidays.
Easter is dependant on the solar calendar.
It is the first Sunday after the first full moon after the Spring Equinox. It can be anywhere from March 22 to April 25.