An event or occurrence that does not happen on a predictable schedule. Since Easter does not fall on the same calendar date each year, it's often called a movable feast.
1.Lit. a religious holiday that is on a different date from year to year. Easter is the best known movable feast.
2.Fig. a meal that is served in motion or with different portions of the meal served at different locations. (Jocular or a complete misunderstanding of {1} but in wide use.) We enjoyed a real movable feast on the train from Washington to Maimi.