That does stink but you don't want to King Solomon your S if it's only another week of it. You could ask if he wants to stay at Grandma's the whole time? But otherwise maybe just leave it and you'll know for next time not to rely on her.

I have to do that all the time with my H, slowly training myself not to rely, not to ask at all ever, just to accept when he suddenly announces he will spend time with one or both kids.

I believe I will see the bounty of the Lord in the land of the living.
Wait for the Lord with courage.
Be stouthearted, and wait for the Lord.