Holiday approvals for most roles in the company is pretty straightforward, for PMs not so much.
Avoid overlapping holidays where multiple PMs are gone simultaneously.
Avoid planning back-to-back holidays where handovers and handbacks are made impossible.
Clarify who has what role during the holiday.
Your goal is to produce something like this so that everyone involved knows when people are gone and who has what responsibility during what period. The warning symbols indicate projects that need special attention. (Sidenote: the concrete case below has both back-to-back holidays and overlaps, which we should avoid; that happened emergently before this gameplan was written, and is a reason why it was written):