To all you assorted Western/Latin Christians out there: Happy Easter!
For we Eastern Orthodox Christians (and the Nestorians and Non-Chalcedonian Orthodox Churches as well), this is Holy Week. I’m not sure about the Uniate churches (the formerly Orthodox churches who are now under the Pope), but they probably follow the Roman date.
The dates differ for two different reasons. First, the Paschalion, the prescribed method which determines the date of Pascha, differs between Western and Eastern methods. While both are based upon the rule “first Sunday after the first full moon of spring,” the Orthodox includes the further proviso that this Sunday also occurs after the Jewish Passover, which in ancient times would actually have occurred on that full moon, while the Jews in Jerusalem were still determining the calendar by observation. Secondly and more importantly, the implementation of the Paschalion differs between the two traditions due to the underlying calendar being different. The calendar of the Roman church, the Gregorian calendar, is known and used worldwide as the common civil calendar. Readers of this blog might or might not be aware, however, that the Orthodox Church still uses the Julian calendar, currently 13 days behind the Gregorian calendar, in determining the Pascha, and a set of traditional rules in which “spring” is defined as falling on “March 25.” Because of all the various elements in play, the two dates, for Western Easter and Eastern Pascha, are seldom the same. This year, Orthodox Pascha is only a week later, but some years it can be up to six weeks later! Please note also that the calendar issue is one of the “campaign issues” of various traditionalist groups in the Orthodox world. Mention the calendar and expect your ears to be busy for quite a long time….
Sometime in the future, we Orthodox will certainly go to an accurate astronomically-based method of determining the date for the Pascha. My own certainty on this point is based in the care that Patristic authors showed for the magnificent creation of God. With our modern tools at their disposal, they would surely have found the real, astronomical date of spring to be a more important determinant for the date of Pascha than the date on a defective calendar which is sliding its way slowly through the seasons.