Your weekend job doesn’t own your weekday nights. Say so before it becomes the norm.

Whatever you accept at the start quietly becomes the baseline. A job agreed for Saturdays can drift into Tuesday evenings one favour at a time, and once the pattern exists it gets treated as the arrangement rather than a stretch of it. Saying what you are available for is far easier before that than after. Nobody is plotting; schedules just expand into whatever space allows them.

Tell your manager now which nights you do not work, before the first request arrives. Put it in writing if there is a system for it.