Many people never think about the date of Easter. This year (2016) most Christians in North America will celebrate Easter early, March 27th. Orthodox Christians will celebrate Easter over a month later on May 1, 2016. The reason why goes back to the early centuries of the church.
So first, how is the date of Easter decided? Well, there is a formula. In the west Catholics and Protestants schedule Easter on the first Sunday after the first full moon after the vernal equinox (the first day of Spring). That formula was decided in the early centuries AD as Christian leaders felt a need to distinguish themselves from their Jewish neighbors and co-religionists.
Scholars today disagree on”the parting of the ways,” that is, the period when the religions we know today at Judaism and Christianity went their separate ways. Some think the divide was complete in the first century AD, others the second, still others the fourth. The point is there is good evidence of close collaboration for centuries between Jews, Jewish Jesus followers and non-Jewish Jesus followers.
It is important to remember that at first all the followers of Jesus were Jews. They kept kosher, observed the Sabbath, circumcised their male sons on the 8th day; in other words they lived Jewishly. But as the Jesus movement grew, it became primarily a non-Jewish phenomenon. There were stresses and strains and ultimately fissures and cracks. It became clear to practitioners of the two religions–if these movements can be classified by the modern term “religion”–that they had different destinies.
The Church Councils in Nicaea (AD 325) and Constantinople (AD 381) set out to define creeds, practices, Christology, and the date of Easter among other things. The first generations of Christians related the crucifixion and resurrection of Jesus to the Jewish Passover, the season when these terrible, wonderful events occurred. The Passover is itself a feast that moves around on our calendar (this year, April 22-30, 2016), though it is stable in the Jewish calendar (Nissan 15-22). Eventually Church leaders decided to sever Easter from anything having to do with the Jewish Passover. By then, it seems, the parting of the ways is complete.
All religions or groups define themselves over against others. This is a natural and normal feature of all groups. Eventually these differences take on the form of a “checklist.” Christians are those who believe X, Y, Z and practice 1, 2, 3. Jews, on the other hand, do not believe X, Y, Z and have a different set of practices. These boundary markers were unclear at first; it was possible in the first century to be a Jesus follower and a Jew at the same time. Over time the differences become clear and stark; now a Jew who becomes a follower of Jesus is no longer a Jew but a Christian.