There is no universal answer to this question. We would recommend making that decision at a student level. There may be some students you really want weekly monitoring for, even during short weeks.
If you just completed PM late in the previous week, and the student has not had time to show more growth by participating in intervention or instruction, it may not be a good use of time to PM during a short week. However, there may be situations where the weekly data is very important for variability in student performance, or regular check-ins when the student's attendance is spotty, or even some behavioral reasons around holidays/breaks. Also, anytime you skip a week, it slows down your decision-making- since you need a pattern of PM to make good decisions about intensifying an intervention.
The key is to make those decisions individually and avoid all-or-nothing, "we ALWAYS PM during a short week" or "we NEVER PM during a short week." The function of progress monitoring is to have regular data about improvement, but it is understandable that there will be times when school isn't in session. The only consequence is to the consistency of the data for decision making; there is no mythical 'ding' to fear.