Bar none, Easter is the single most important holy day of the Christian Church. It celebrates the resurrection of Jesus Christ, the central event in Christianity. To Christians, the resurrection backs up Jesus’ claim that he had the authority to die for the sins of the world and the power to come back to life again. It also gives hope to Christians that they too will experience a resurrected life in heaven.
The exact day of the year that Easter falls on is very confusing, and the logic seems pretty old-fashioned in this digital age; it’s based on the lunar calendar and tied to the start of the solar spring. But the Western Church (Catholic and Protestant) continues to observe it based on the rules of long ago — that it falls on the first Sunday after the first full moon after March 21 (the vernal equinox). It can’t come before March 22 or after April 25.
In contrast, Orthodox Churches wanted to tie Easter to Jewish Passover, given the relationship between Passover and the day of Christ’s resurrection. Because the Jewish calendar determines the date that Jews celebrate Passover, Easter for Orthodox Churches can vary by as much as five weeks from the Western Church.
No one knows for certain where the term Easter came from, but one theory is that it’s derived from the Anglo-Saxon goddess Eostre, who was connected with fertility and spring. If so, Christians named their high holy day Easter aiming to replace the pagan celebration of spring with their own holiday — like they did with Christmas. Easter is also often known as Pasch, which comes from the Hebrew word Pesach, meaning “Passover.” Some Protestants prefer to call it simply Resurrection Day to remove the commercialized baggage that they see associated with Easter.
In addition, the Easter bunny has pagan origins and has no real connection with the Christian celebration, although some churches use eggs as a metaphor for the new life Christians receive because of the Resurrection.
Source: What and When Is Easter? By Richard Wagner, Christianity For Dummies