A fellow National Puzzlers' League member posed a question to the mailing list: Where does movable feast come from? The answer isn't "the title of a Hemingway novel." A flurry of word geeks chimed in, and I guess it's no surprise that Wikipedia gives the full scoop. Christian holidays tied to a particular date, such as a saint's day or Christmas, are immovable feasts. Christian holidays that can shift around the calendar, such as Easter, are movable feasts. Now you know.
Speaking of movable feasts, the rules for calculating Easter's date for any given year are deliciously complicated and worth a look. Über-mathematician John Conway published the algorithm, an adjunct to his system for determining the day of week of any given date, in the second volume of the original Winning Ways series. I don't have the new versions handy as I write this, but I'd guess it's now in Winning Ways Volume 4.
