Originally Posted by Cat Herder:
You are not obligated to do anything.
Honestly, in your position, I'd suck it up and honor my original schedule. 
I can say that because I have in the past. It is about your business reputation as a dependable provider. 2 days notice, especially weekend days, seems unfair IMHO. It would be very difficult to find back-up care on a holiday.
From here on out I'd recommend taking all federal holidays off with pay. Most providers I know, including myself, already do. 
I agree with this. It definitely does suck and I've been there a few times and I would casually mention to mom that her child will be the only one there but from my experience the SAHM's who have their children in full day care on a holiday aren't going to care much! I would just suck it up and learn from the experience!