It's bloody hard, isn't it, @Sunshineandflipflops. Even worse when there's an OW involved.
Both this year and last year my ex and have negotiated a Christmas holiday split where he gets the second week (so he can get on a plane to visit his parents the second he picks up DS, lest he has to actually look after him himself for a week). So I allow him to see DS for several hours on Christmas Day - after we've had lunch. I had been expecting an alternate year thing, but he obviously values the week at his parents bit more.
MrSG now has an alternating Christmas arrangement with his DC (theoretically) but his ex will pull any old dirty trick to prevent it. He's supposed to have the Christmas Eve and part of Christmas Day (including dinner). Then she's picking them up to have them and returning them at some point on Boxing Day. But I guarantee that she will try to ensure she gets Christmas Eve and all of Christmas Day, with him doing the driving.
Last year they were supposed to be doing 'one last joint Christmas' with the DC but a couple of days before Christmas she started being a total shit. Turned out that what she really meant was that she wanted him to pay for one last Christmas in its entirety but not to be inconvenient and want to spent any time with the kids (unless she wanted rid of them).
First she tried to get rid of him on Christmas Eve/Christmas morning but he refused. Then she announced that she had arranged for her and the kids to have Christmas dinner at her mum's. But, crucially, she wanted him to pick them up from her mum's at 5 so that she could go to her boyfriend's (the potato she had an affair with). He refused, pointing out that if he was being excluded from Christmas dinner, he wasn't staying sober so he could provide taxi services and look after the kids only when it was convenient to her.
Then on Christmas Eve she decided that, actually, Christmas morning would be cut short because she wanted to take the kids to her boyfriend's house at 10am and then on to her mum's from there. He was so upset and stressed and refused to have that happen, and she was really horrible about it.
He had Christmas dinner and spent Christmas night with me. But it wasn't a nice situation at all. She's really a piece of work (manages to make my horrible, abusive ex seem reasonable regularly - which is a feat) so I'm expecting that she's not going to honour the agreement she's made (because she really does believe that she should get Christmas - the whole of it - with the kids every year).