I see two choices here:
1. You act as if your plans are set. Get your items crossed off your to do list. Don't add anything else to your calendar. Then if no invitation is forthcoming, you have a free night to rest and relax.
2. Decide that it's too late, and if an invitation is made, decline and say you are sorry, but it's a bit too last minute for you.
I think either choice is valid and polite. I'm not a fan of last minute stuff, but I can go with the flow with tentative plans since I can gear up for whatever it entails. Then if it doesn't come to fruition I'm just as happy to have the free time.