Learning to say “I’m not available that day” at your first job saves you years of overcommitting.

A first job is where the habit gets set, for better or worse. Saying you cannot do a particular day, early and without drama, teaches both you and everyone around you that your time has a shape. Leave it unsaid for a few months and the opposite gets established, and that version follows you into every job after, because by then you believe the asking is rude.

Use the phrase once this month, exactly as it stands, with nothing added. Not available is enough, since a reason invites a workaround. You will notice it lands as ordinary scheduling rather than as a problem.