Mmmm this is a tricky one.
But I think there must be more to the story.
To me it sounds like a very nicely put email, that if you responded and said " this upsets up and we want to continue" that it's probably what would happen.
We have our family Christmas before the actual day! It works out fabulous. It means on Christmas day we get to do the whole present thing at our place, we invite my immediate family for brekky and then we have the ILs thing in the arvo.
I think they are trying to do what's right for you.
Sorry you're feeling so unhappy about it.