Holiday schedule negotiation starting - advice appreciated

We agreed loosely that ex gets Thanksgiving, I get Christmas, but never put specifics in writing (huge mistake, I know). Now she's asking about Christmas Eve (when kids will open presents) vs Christmas Day (my legally scheduled day).

I want the Eve because that's the tradition we had together, but I also don't want to be the guy fighting over literal hours on Christmas. How do people usually split this? And should I try to get it in writing for next year?