Good luck, it's going to be a long fall. I don't count any holidays until we pick up SD and she's in our car, and even then BM sometimes would start blowing up SD or DHs phone until she got her back early. 

I get wanting to spend the holidays with your children. But some of these women can't get past their own selfish desires for what they want rather than but is best for their own kids. Most holidays you can celebrate at a day near to the real holiday except for Christmas and 4th of July. BM didn't care about 4th of July until she found out that my husband really loved it now she demands to have SD on that day too.

We have never had SD on Christmas day even though my DH is entitled to it every other year. It's always some reason or another or some last minute change and I used to get so frustrated because my family ended up being at the bottom of the priority list of people to see and I will have to call them with updates for our visiting schedule based off of bm's whims. I was so mad why did this b**** that I didn't even know get to decide when I saw my family?? I put my foot down and told my DH I am not planning around maybes anymore.

Our BM kept SS for all holidays: Easter, SS' birthday, Thanksgiving, Christmas Eve/Day, New Years Eve/Day until SS was 7. BM always made these holidays hell because she claimed that she had traditions and DH having SS would interfere with those traditions. Like DH wanting to have SS, but never getting him didn't interfere with our holidays at all. BM did try to offer DH a Christmas Eve or two when SS was little, but she always cancelled last minute or demanded that if DH wanted Christmas Eve with SS, then he would do it at BM's parents house with BM and her family, so they didn't miss out too, so DH just gave them up. He wasn't about to play "happy family" with BM and her folks. 

The court finally ordered holiday visitation in our long distance CO after BM moved. They alternate school breaks (Thankgiving and Spring break) and split half of Christmas break yearly. It hasn't always been easy. BM demanded that the CO state that SS be dropped off from all holiday visitations two days prior to school starting so they can celebrate with SS too, cutting our visitation short every time... 

In 2019, we had our first Christmas with SS (he was 7 at the time) and my family went on a huge family vacation to Disney. It was my parents, my brother, his GF, us, and SS. You better believe that BM tried to mess with that trip/Christmas. We let BM know about the trip in April, because per our CO, she had to approve all out of state travel. She approved. Then in December, two weeks before the trip, BM backpeddled because she was mad that DH took her to court for more visitation and joint legal. BM claimed in December that she promised SS he could attend a holiday party with his friends the day of pick up for our trip... a kid's holiday party was prioritized over Disney. We took BM to court and paid thousands just to resolve this specific issue because we had nonrefundable hotel and park ticket expenses that added up to thousands. We told BM we would sue for the full value of the costs if she prevented the trip. BM just smuggly grinned. The judge compromised and allowed SS to attend half the party, but gave us an earlier pick up than BM wanted and ordered that SS attend the trip. Come to find out, the party was at BM's GF's work... there were no kids present at all. It was an office party. DH was livid. Then BM and GF, together and separately, proceeded to call SS multiple times per day while we were at Disney and DH did his best to run interference and prevent the constant calls. He sent many to voice-mail and told them we would call them back. Yet when BM has SS for Christmas or any other holiday, DH always calls SS to wish him a Merry Christmas/Happy Thanksgiving/etc. first thing in the morning, trying to be courteous, allowing BM to fully enjoy SS for the entirety of the holiday. But not BM... 

So yes, to answer your questions, HCBMs will always rewrite history and reality to suit their wants. 

I'd change the Christmas celebrations to another day and drop the rope on trying to argue with a HCBM.  Trying to agrue with these people is like wrestling with a pig.  You both end up covered in mud but only the pig enjoys the experience.  

I know its annoying but maybe if you stop wanting 25 December, HCBM will get bored with being a witch and let you have the day in up coming years.  Maybe if you make the feast of kings your special day as an example.  That's when the kids in Spain get their gifts.

On other thought, no kid is going to object to two Christmases.

HCBM and DH have a terrible relationship, and she does the best that she can to sabotage anything and everything.  She leaves him off of school documents, did some severe PA with SS when he was little, makes everything as difficult as possible, in whatever ways she can in hopes that DH and the SKs will have a terrible relationship.  She's failing more and more as the kids realize that it's all about what she wants and not about taking care of them.

Which gives me last Thanksgiving as an example...  It was the first time EVER that she was going to be alone.  She's in a relationship with a married couple and they went away last year without her, and the SKs were scheduled to be with us for the holiday.  The nght before she was supposed to drop off SS (other SS now lives with us FT), she reported that she had been exposed to COVID and had to quarantine--which meant that SS had to quarantine, as well.

DH asked for documentation about her exposure.  She's AD military, so there would have been something or someone to validate.  She refused.  She called on Thanksgiving day and left a message (because we don't answer calls from her) that he could come pick up SS for the day, if he wanted, which then made it his fault that SS wasn't with us for Thanksgiving, and of course there was no makeup time. 

She reported another exposure earlier this year when she was going to be alone again, but we just let SS come and quaratined him in his room for the ENTIRE 4 days.  It was miserable for him (he was on electronics restriction for part of the time, too!), but he figured out who was causing the problem and who wasn't.

All that to note my agreement with the others that you can never count on a SK for anything when you're dealing with a HC bio parent.  They rewrite history, they create their own rules and just generally do what they want.  Sorry you're in this boat, too.