Take the issue of the date of Easter, for example. This year, westerners celebrated this festival a week earlier than the Eastern and Oriental Churches. Which westerner could say why? Easterners know very well. It has to do with history, about which we in the west don’t really care very much. We’re not really so interested in the past as we are in starting with a clean slate to shape our own futures. Yet we have little understanding how much our past has already shaped who we are and why we do things in certain ways.
Briefly, then: at the Council of Nicaea in 325AD, leaders from across the then-Christian world agreed on a formula to celebrate Easter on the same date: the first Sunday after the first full moon following the spring equinox (March 21); but always after Jewish Passover, as Jesus and his disciples had celebrated Passover the night before he was crucified.
However, the Julian calendar, which had been in use since 45BC, was a solar calendar. The Catholic Church adopted the Gregorian lunar calendar in 1582 in order to compensate for the loss of days which built up over a long period. Most countries have since followed suit, but the Eastern churches have held to the Nicene Council formula, when Easter falls anywhere between April 4 and May 8. For Catholics, and for the West with them, Easter falls between March 22 and April 25. Sometimes the two Easters fall on the same dates, as in 2010, 2011, 2014 and 2017. But this won’t happen again until 2034!
Source: Jeff Fountain - Weekly Word eNewsletter, 1 May 2019, "Vibrant Celebration"
https://us9.campaign-archive.com/?e=0b86898e11&u=65605d9dbab0a19355284d8df&id=c06ec3e640