UPDATE 2014-01-15: The attached .zip file now contains a Mathcad Prime 3.0 worksheet (.mcdx) and—for those of you who are still using earlier verisions of Mathcad—an Adobe Acrobat printout (.pdf) of the worksheet so can see how it is put together.
This worksheet calculates the date for Easter Sunday using a modified version of the so-called "Meeus/Jones/Butcher" Gregorian Easter Algorithm that was presented by Ian Stewart in the March 2001 edition of Scientific American. The Gregorian Calendar was introduced in October 1582, so this calculation is only valid starting in 1583. Tidal friction is gradually slowing the Earth's rotation so at some point in the distant future this algorithm will require revision. We won't be around to do it.
BTW, this really isn't a puzzle or a game, but this seemed like the best section for this worksheet to reside.