Telling a coworker “I’m not covering this time” is allowed. Covering every time becomes your job.

A favor done enough times stops being read as a favor. It becomes the baseline, and then the day you cannot do it, you look like the one causing a problem. First jobs are where this habit gets installed, because you want to be liked and one shift seems small. Three months later your Saturdays belong to whoever asks first.

Say no once this month to a cover that does not suit you, and keep the sentence short: I cannot take that one. No reason, no apology spiral. You will find out fast that it is an ordinary thing to say, and your schedule stays yours.