As a Mennonite I never really thought about the date for Easter and why we celebrated it when we did. It was one of those questions I didn’t know to ask and only became a small curiosity after I started to celebrate the holiday on a different date as an Orthodox.
Fortunately, there are those at my parish willing to answer those questions I don’t even know to ask. An older gentleman named Joe went through the effort to copy two different references on the topic that explain the divergence of practice between the Orthodox East and the Roman West.
The regulations deciding the date for the celebration Easter were established by the First Ecumenical Council in 325 A.D. There were three criteria, as stated in the booklet, “The Date of Holy Easter” published by the Syrian Orthodox Archodiocese in 1963:
1) “That Easter must always be celebrated on a Sunday”.
2) “That Easter must never be celebrated on the same day as the Jewish Passover”.
3)”That Easter should never be celebrated on or before the vernal equinox of any year”.
Those three criteria were unanimously decided by the church. The Church of Alexandria, with its “noted astonomers,” was picked to announce the date to the Church of Rome every year and Rome given the responsibility of passing that along to the other Churches.
All of Christendom celebrated Easter together, even after the Great Schism, based on the three criteria—that is until the Roman Catholic West decided to unilaterally, in 1582 under the direction of Pope Gregory, adopt a new calendar and disregard one of the three criteria established by the Ecumenical Council in 325.
So, in short, Papal supremacy is why the West (including Protestants) celebrate when they do and the First Ecumenical Council is why the Orthodox celebrate when they do. Of course it does get more technical than that. I’ll send anyone the pamphlets if they are interested in the answers to questions they never thought to ask…