I have to say this isn't uncommon, unfortunately.
Before my Ex and I split up, our 'tradition' was that we went to his parents from late morning to mid afternoon on Xmas Day, kids would open presents, his mum would do a few buffet type nibbles, then we'd come home and have our Xmas meal early evening (I have no family, his lived a 20 min drive away). Since we split, I've continued this as it worked well for all of us, and it means we both get to spend part of Xmas Day with the DC.
My bf also has DC, he has them 2 nights a week - when he got divorced (fairly recently) he was told that the courts would NOT make an order over Xmases, birthdays etc and it would be for the parents to arrange between them. His Ex is pretty ignorant and selfish, and has therefore elected to stick rigidly to the regular weekly contact arrangements at all times, including Xmas, birthdays, fathers days etc. It means that he won't get to spend Xmas with them for another 6 or 7 years. The DC are quite young, are reluctant to rock the boat, so whilst they'd probably like to see both parents on Xmas Day, it's unlikely they would speak up and say so.