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ETA: Easter is the Sunday after the Paschal Full Moon. The Paschal Full Moon may occur from March 21 through April 18, inclusive. Thus the date of Easter is from March 22 through April 25, inclusive. The date of the Paschal full moon is determined from tables, and it may differ from the date of the the actual full moon by up to two days. This definition, along with tables, etc. may be found in "The Explanatory Supplement to the Astronomical Ephemeris and American Ephemeris and Nautical Almanac". This definition that uses tables instead of actual observations of the full moon is useful and necessary since the the full moon may occur on different (local, not UT) dates depending where you are in the world. If the date of Easter was based on local observations, then it would be possible for different parts of the world to celebrate Easter on different dates in the same year.
... That's why the date changes for easter every year... ion the Catholic church the date is calculated from the Paschal full moon in Rome, ie the Vatican and because the catholic calculation tables have been used since the introduction of the gregorian calendar (prior to the reformation and lutheran split) so most other christian religions have kept the formula. Orthodox religions are more inline with the Jewish Passover and therefor that is why Greek Orthodox and the other eastern churches can have their easter celebrations up to 2 weeks different. Also there are some other feast days in the catholic church that are not fixed but shift depending on when easter falls... these usually have something to do with the life of jesus, (and to a lesser extent mary).