Northwesten, not sure about where you live, but here daycares are run like a business. If you decide not to bring your child on a scheduled day, then you pay. Why should they be out the money because you chose not to bring the child ? Daycare's also are paid for vacations, which I DO NOT agree with. They are a service business, and if not open for the service, nor should they be paid. Costs twice as much if the parent doesn't take that same week off, as they have to pay for the daycare on vacation, plus a daycare to watch said child so parent can work. Not fair!
To the OP, WOW! I think I'd blow a gasket, not only the the BO (obviously having a bad day, but directed it at you), but the MIL overstepped her bounds. And what's up with your daycare? Why would they allow her to cancel, she is not a primary parent? Around here, you have to be on the list to pick up the child, and if canceling anything, parents only have that authority.
Don't roll out the welcome mat for MIL, let her feel like an imposition. After all, YOU didn't invite her. Plus, call daycare and let them know that she is NEVER to do something like that again and that the child WILL be there as scheduled.
Now, have a nice chat, LOUDLY if needed, with your darling husband. The woman he needs to stand by is you. The woman whose emotions he needs to concern himself is you. I think that's what those pesky VOWs were all about. Perhaps a refresher of them is in order?