Hoping that you have already confirmed with her that this day IS owing? Because I agree. She took off the one day you were closed, fine. But for a number of reasons, all listed above, she still needs to pay for the other day.

It's not splitting hairs. What if you have all 5 families choose to do this in one week? There goes 175 bucks! Not 35.
Be calm, firm and if need be, show/email her the part of your contract that states "If I am open, you pay".