My current husband and his ex (and I) get along which makes things TONS easier.
When you have a pissed off ex it's hard, they take pleasure in turning the knife and it can be REALLY fucking hard if they don't think twice about using the kid as a weapon.
If the ex is being a baby seriously try to cultivate the attitude, "Whatever causes the kid the least stress, fuck it, it's just another day." You can have a beautiful turkey dinner any day of the year, you can celebrate xmas ANY time in December (my coven, for example, is celebrating Yule on Dec. 6). It's not the DATE that matters, it's the sincerity of the feelings.