Or, the real reason why this year the vast majority of Eastern Orthodox Christians are celebrating the Feast of Feasts five weeks after the rest of Christendom. From the always informative blog De unione ecclesiarum:
Although for most Christians Easter is just around the corner, for the Orthodox Church Lent began this week; there is a five-week disparity this year between the dates of Easter (April 27th for the Orthodox, March 23rd for everyone else). When I was younger I asked my mother why the Orthodox Easter and the Protestant and Catholic Easter fall on different dates, and was given the following explanation: for the Protestants and Catholics, Easter falls upon the first Sunday after the first full moon after the vernal equinox; for the Orthodox, Easter is the first Sunday after the first full moon after the vernal equinox after Passover. As a rule of thumb, I have found this explanation always to work (e.g., Passover begins this year on the evening before April 20th, which also happens to be a full moon); however, I have also learned that, as an explanation for why the differences exist between the Orthodox Church and other churches in their calculation of the date of Easter, it is erroneous. The difference between the dates of Easter arises from the fact that the Western churches calculate this date according to the revised, Gregorian calendar while the Orthodox Church calculates Easter according to the old Julian calendar. That is to say, all the churches observe the rule, established by the Council of Nicaea in the fourth century, that Easter be celebrated on the first Sunday after the first full moon after the vernal equinox; but they identify differently the day on which the vernal equinox occurs. Although the vernal equinox — the moment when day and night are exactly equal, or, to speak in Ptolemaic terms, when the sun, in its annual journey through the zodiacal belt, crosses the celestial equator, marking the beginning of Spring in the Northern hemisphere — actually occurs on March 20th this year, the Orthodox Church reckons “March 21st” as a fixed date for this astronomical event, and it reckons this fixed date according to the Julian calendar. Currently, the Julian March 21st is the Gregorian April 3rd, that is, roughly 13 days later than the astronomical equinox; over time, the discrepancy will continue to grow, at the rate of about a week per millennium, so that, if nothing else changes, Orthodox Christians in the year 6008 will be celebrating Easter in late May or June.
See also this very helpful article by the late Orthodox canonist Archbishop Peter (L’Hullier).