My hill to die on with my husband's family (we used to live near by and they were who we did most of our Holiday-ing with). My MIL expected us all to get together for the "upcoming holiday" but details were not decided until a day or two before the event (and sometimes, not on the holiday itself). One year, I said to my dh* that if we didn't hear about Easter plans at least two weeks before the date, we were doing our own thing.
On the Sunday two weeks before Easter, a good friend invited us to come to her house to celebrate Easter with her family. I checked with my dh and he said it's a great idea, so we agreed. At some point, I think maybe on Holy Thursday, we were told the details of Easter at my in-laws. We informed them that we would be able to stop by for a bit in the afternoon, but we weren't able to come for lunch, because we would be with the H family. That didn't go over well.
On Easter Sunday, we spent a great time with the H family, then headed to my in laws. Everyone was there already, the food was eaten, and we got a few snide comments from one brother. We stayed for an hour and left. (Mainly because the conversation was really not kid friendly and I stood my ground on that one as well.) It felt good.
The other hill I died on was the Gift Exchange. We have permanently removed ourselves from it and finally gotten my MIL to understand that no gifts will be coming for us. (We aren't cheap, it's just that we want to focus on the Birth of Christ rather than the gift giving.) The last time we participated, we sent gifts out timely to the four people we drew, and our kids got gifts, in August, when we were home to visit.