Your availability should reflect your life, not somebody else’s habit of asking.

Availability drifts toward whoever asks most often. In the absence of a decision, other people’s habits quietly become your calendar, and because each request seems small on its own, there is never an obvious moment to object. What you end up with is a week shaped by other people’s patterns rather than your own life.

Decide what shape you actually want your week to have, including the parts that are not work. Then let people learn it through what you do rather than through a speech. Saying that a particular evening is taken, without further explanation, is usually enough.