Your weekends do not need to be available simply because your workweek ended.

Free time is not the same as unclaimed time, though other people read it that way and often so do you. The moment Friday arrives the calendar looks empty, and every request seems reasonable because nothing is already written in the space. Rest that has not been decided on in advance tends to be given away in small agreeable pieces. Availability is a choice rather than a default.

Put something into the weekend before anyone asks, even if the something is nothing in particular. When a request comes you are then choosing between two real things. A plain no, with no reason attached, is allowed.