Last minute illness may cause us to cancel reservations for a Sunday arrival. If this is necessary, yet hoping its not, can this be done by calling the hotel directly after MS closes on Friday for the weekend?
Can't answer for sure and suggest you call MS on Friday and ask them.
Hope you don't have to cancel at all. I'm sure you know if you cancel by Saturday the points will go into the holding account. If you don't cancel until Sunday your day of arrival, you'll lose the points altogether.