Definition: In the strictest sense, a moveable feast is a Christian feast day whose date changes each year because it depends on the date of Easter, which is itself a moveable feast. (See How Is the Date of Easter Calculated? for more information.)
By extension, the term moveable feast is used for any event on, or related to, the Christian calendar that varies from year to year. Thus the Baptism of the Lord, while not related to the date of Easter, is celebrated on a different date each year, and Mardi Gras or Shrove Tuesday, while not officially a Christian celebration, is tied to the date of Easter, and both are colloquially referred to as moveable feasts.
Pronunciation: ˈmoōvəbəl fēst
Alternate Spellings: Movable Feast