[PenguinTrax]

I wouldn't risk not going into work - that would be definite grounds for termination not to mention you would be putting those employees that did show up that day in a bad situation.

I would explain to your manager the situation (grandmother's birthday, etc.) and ask if there is any way you can get off (at least) for Thursday. That way you could join your family on Wednesday, which is better than not at all.

Alternatively, find someone else that's wasn't originally scheduled to work those days and switch with them.

It seems odd to me that a Montessori school would be open on a holiday.