From bitter experience if the ex is the sort of person who can't be trusted to keep to her word, who threatens cancelled contact if you don't do what she wants, who'll never reimburse you for money wasted etc., then I'd apply to court now for what I think is called a specific contact order (I may have the terminology wrong).
That may seem an extreme measure to some of you but you can't reason with someone who's unreasonable - and only you know how fair, honest and co-operative - or not - the ex is.
In our case we "agreed" a summer holiday 7 months in advance - complying exactly with the ex's demand to book very specific dates (we weren't allowed to go a day earlier etc). Having done that, which wasn't easy in high season on a limited budget, we were informed, by text late in the afternoon of the day before we were due to leave that the children wouldn't be coming. No apology, no shame, no explanation whatsoever. And then the ex refused to answer her landline or respond to any susbsequent texts. We basically poured around £7-800 down the drain as we'd otherwise have gone at a different time if the stepkids weren't coming. DP was devastated, I was furious - the holiday left a very sour taste in the mouth and was overshadowed by all this. What was worse was that I just knew - well, 99% gut feeling - she'd pull a spiteful stunt like that as had past form for cancelling agreements and mucking about with contact. I'd pleaded with DP to get a contact order prior to our trip (for that, and also the ongoing messing about over contact) but he dragged his heels 
and left it far too late.
I should think a judge would take a very dim view of your situation. You have her agreement in writing and if she subsequently wanted to book something she should have checked before doing so as DP had already advised her that there was a chance he'd come back on the 24th. Threatening to withhold contact if DP doesn't back down won't go down very well either. Only thing is, and again this is from our eventual experience of court orders, is that things can move very slowly and I'm not sure you'd be able to get anything stamped before Xmas. It would however, do no harm to at least enquire if such an order for a specific event like this can be granted - in theory, and in enough time. Long term, your DP should certainly consider a contact order anyway - these can be worded to cover off important occasions like Xmas - so he may at least be able to do as much as he can in regard to next Xmas (though unfortunately contact orders are never a 100% guarantee the ex will play fair).
Again, IME, the more the NRP buries their head in the sand, tries not to "rock the boat" and so on, the more an unreasonable ex takes the piss. When all our contact troubles - and definite alienation - began I was very fearful it would escalate unless it was nipped in the bud asap - via legal means once appealing to her "better nature" had failed - and unfortunately that turned out to be true. If I could turn the clock back I would have been far far more insistent with DP that he applied for a contact order much sooner than he did. The mess he/we have had to unravel after years of this was so much more difficult than it should have been and has had a significant impact on all sorts of things - family relationships, our relationship, our finances etc.