I am looking forward to Easter this year, not only for the usual reasons (You just can't get better liturgy anywhere!) but also because this year, Christians throughout the world will celebrate Easter on the same day. The Catholic (and also any other Western Christian) Easter and Orthodox Easter will both be on April 8. This certainly does not occur every year. Next year the two Easter celebrations will be over a month apart, with the Catholic Easter on March 23 and the Orthodox Easter on April 27. This is due, primarily, to the differing calendars used by the Eastern and Western Churches.
"Easter Day is the first Sunday after the full moon that occurs next after the vernal equinox." (Intrestingly, when I googled for the official definition, the first site I found was The Astronomical Applications Department of the US Naval Observatory.) Things get a bit tricky here, because this isn't the actual equinox, nor the actual full moon, but a legal fiction. The equinox is always defined as March 21st. That's fine we can go along with that, right? And the full moon is defined as the 14th day after the new moon (which I suppose is also mostly a legal fiction). Anywho, there's the date.
I had understood previously that the Eastern Church actually used different rules to calculate Easter, based on the Jewish calendar, but that does not actually seem to be the case. I must have been mixing up the earlier Easter controversy with the current one. In fact, the algorithm is the same: the first sunday after the first full moon after the vernal equinox. However, the Orthodox Church still uses the Julian Calendar for its liturgical celebrations. As the centuries have gone by, since 1582 when the Gregorian calendar was first officially adopted (although not worldwide), the dates between the calendar have gotten more and more off. Moreover, the Orthodox Churches use different algorithms to designate the official vernal equinox, the official full moon, etc. There have been some efforts in the Orthodox Church to revise their calendar to coincide with the Gregorian calendar, but these have been resisted by many, as they are seen as "giving in to Rome."
Anyway, that's what I've been learning about today!