Ever since the Council of Nicea "fixed" the date for Easter, it has almost always occurred at different dates from one year to the next. By the council's definition, Easter occurs on the first Sunday after the first full moon after the spring equinox (which we call the beginning of spring).
As a result, the Easter date can vary by more than a month. This year, it occurs very early (March 23, 2008). The reason is that equinox took place on March 20 at 5:49 Universal Time (UT) - that is, March 19 at 10:49 Pacific Daylight Time (remember, we just switched to PDT on March 8) - and the first full moon after that occurs today (Friday, March 21, which also is "Good Friday" this time) at 18:40 UT (11:40 PDT, about an hour from now, as I write this). The next Sunday occurs the day after tomorrow - so that one is the Easter Sunday.
If the full moon had occurred even just one second before equinox, we would have to wait a full lunar cycle to the Sunday after the following full moon (29.5 days from one full moon to the next, approximately, called a Synodic month) for Easter to happen. If that full moon were to happen on a Monday, we'd have to wait an additional six days to get to the next Sunday - Easter. You do the math for each particular year of your interest.
I'm not addressing the religious aspects of Easter here. It's obvious to me that the Easter date is totally artificial and man-made. But it's interesting to me, because it has to involve astronomical observations and calculations (as most of you who read this blog know, one of my hobbies is Astronomy). The whole thing is an outgrowth of man's preoccupation with the concept of "time" (another anthropocentric idea).
If you're interested in the more detailed aspects of the lunar orbit and the concepts behind our definition of time, an excellent source of information is the "Observer's Handbook", published by the Royal Astronomical Society of Canada. For quick and detailed references, this handbook is used worldwide by amateur and professional astronomers alike. I took the times and dates information above from that handbook.
