ˌmovable ˈfeast noun [countable] British English

1. informal something that happens at different times, so that you are not sure exactly when it will happen

2. a special religious day, such as Easter, the date of which changes

[TahlilGaran] Dictionary of Contemporary English ▲

movable feast
something that happens often but at different times so that you are not certain when it will next happen.
They usually have a party at some point in the summer but it's something of a movable feast.

[TahlilGaran] English Idioms Dictionary ▲