Hey, wait a minute - Easter Sunday is the first Sunday following the first full moon following the Spring equinox; a combination of lunar and solar markers.
Unlike every other Christian holy day, Easter is still based on the lunar calendar. That's why it falls on different days every year, and why it almost always coincides with Passover.
Easter, termed Easter, is sort of a bastardization of the celebration of the resurrection of Christ. Alot of Celtic pagan traditions worked their way into some Chistian celebration in order to more smoothly assimilate pagans into the church after they heard the Good News. The Easter Bunny and the name Easter (a modern pronunciation of the name of a pagan spring goddess) are leftovers from this assimilation. However, as far as scholars can tell, the resurrection of Christ was celebrated by Christians as early as a hundred years after the event took place.
The fact is is that our salvation is dependant on the events of Good Friday and Resurrection Sunday (which is, by the way, why most Christians worship on Sunday instead of the Sabbath) making Easter - or Resurrection Sunday - the most important holiday on the Christian Calender.
Constantine never knew anything of "Easter". The holiday to him was "Pascha", the same Greek word used for the Passover of the Jews and obviously derived from the Hebrew pesach. (The same word is used in Latin and all languages derived from it.) You can prove that simply by reading the decree of Nicaea on the dating of Easter in the original Greek; it's online.
The Anglo-Saxon word "Easter" came to be applied to a Christian religious holiday in England and Germany long after Constantine was dead and buried.
Easter, coming from the pagan religions of the east
It had nothing to do with any "pagan religions of the east".
is tied to the solar calendar. Passover is based on the lunar calendar
The calculation of the date of Pascha/Easter is complicated, but it isn't strictly solar. It's a hybrid system that involves both the solar calendar (Gregorian in the west; Julian in the east) and a heuristic lunar equivalent. That's why it falls on different dates each year.
(It's the first Sunday after the first new moon after the vernal equinox, March 21; however, the date of the "first new moon" is calculated using a system of tables, not by actual astronomy.)