From Longman Dictionary of Contemporary Englishmovable feastˌmovable ˈfeast noun [countable] British English 1 informalDL something that happens at different times, so that you are not sure exactly when it will happen2 TMCa special religious day, such as Easter, the date of which changes each year
Examples from the Corpus
movable feast• A birthday is a movable feast as the round of years is wholly unrelated to the seasons. 