a movable feast的用法和样例：
- Christmas is fixed,but Easter is a movable feast.
- Christmas is fixed, but Easter is a movable feast.
- We were all surprised at that, and Masson remarked that we'd had a very early lunch, but really lunch was a movable feast, you had it when you felt like it.
- Thanksgiving is a movable holiday.
- A movable electrical contact, as in a rheostat.