movable feast
noun [ C ] uk/ˌmuː.və.bəl ˈfiːst/ us/ˌmuː.və.bəl ˈfiːst/
节期因年而异的宗教节日
a religious holiday that is not on the same day every year
Easter is a movable feast. 复活节的节期因年而异。
informal无常事，并非一成不变的事
something that can happen at any time or place that suits people
Lunch is officially from 12.30 to 1.00, although it tends to be a movable feast. 按规定午饭时间从12:30到1:00，而实际上往往是不一定的。