When Exactly Is Easter?

In the early church there was much discussion and debate as to the proper day for the celebration of Easter.  The basic concern was whether to celebrate it on the day of the Judaic Feast of Passover or on the closest Sunday thereto.  This discussion was ended in 325 A.D. by the decision of the Ecumenical Council of Nicaea, which decided the Feast of the Resurrection of our Lord Jesus Christ would be celebrated on the first Sunday following the full moon after the vernal equinox. 

This decision and particularly the celebration of Easter on a Sunday was based on the fact that it was a Sunday on which Christ rose from the dead -- the second day of Passover of that year.

The Feast of Theophany and related observances are celebrated by precise chronological (solar calendar) equations and are not variable, no matter on which day of the week they may fall.  Likewise, the Feasts of the Holy Mother of God, the Holy Cross, and the Church are celebrated by using the solar calendar. 

Easter, however, and the religious observances connected to it, are calculated by the lunar calendar and like the Passover, are variable within a 35-day period.  Therefore, Easter may fall between March 22 and April 25. 

The variation of the celebration of Easter between the Eastern and Western churches is strictly related to the calendar and the equations thereof, and in no way are theological.