I think many of us are running into this. We've been back in for two weeks and I've lost four days in a four-day-a-week class between MLK and weather-related incidents (yay building flooding too!). Thankfully it's early enough I can make reasonable adjustments.
At my university, we are given relatively wide berth to reschedule and work around campus closures and based on the question, it seems the situation is similar, so here are some of the possibilities I'd see:
- Take home exam with regularly scheduled Monday
Not optimal, because they'd end up just cheating as it will be hard to rewrite a test designed for classroom-taking for home-taking in such a short time, but essay-style tests may work okay (doubt that's the case for a math course, though)
- Reschedule the test day on the weekend.
It may seem a bit dickish, but you could reschedule for Sunday and then handle the handful of no shows individually (this presumes that the weather is okay by Sunday, Saturday looks to be a no-go all over). Students will kill you on evals for this.
- Move the test to Monday and have a special session to cover Monday's material
Presuming one of the remaining days is a review-like day, cancel it to fit in the rest of the days. The review day will then be rescheduled as a special session at a time that the vast majority of students can make (perhaps consulted via a survey).
- Cancel the test and integrate into the final
This presumes there is a final. Some students may balk at this, and since it would involve the modification of the syllabus may require approval of higher ups or be subject to other university rules.
Some universities may have more codified policies that explicitly add on extra days, in which case you should modify the schedule in accordance with that policy. No doubt that a department head or dean would remind you of such a policy in advance (our provost did, which is to say, we were encouraged to hold virtual classes, but were reminded that we were free to handle the situation — including rescheduling classes outside of normal hours — in the ways that best fit our courses)