Solost I hope you can confirm that when he gives his diluted version of the truth, you'll allow a pause and add "Well, that's not the full picture, because Daddy met another woman earlier this year and he left to be with her in August"? You do intend to say that, don't you, in front of him and them, immediately?
His "lite" version of the truth doesn't surprise me one jot. The fact that he's put this off until Wednesday now, is simply playing for time in the hope that you'll back down.
FWIW, I do understand why you want him to see and hear your DCs pain, because if nothing else, this might finally bring home to him what the consequences are of an idiot man who couldn't tell the difference between a crush and real love, but stick with this plan only if it doesn't prolong the agony with your DCs. This current plan only leaves 2 clear days before Christmas and it is truly disgusting that he is putting his own needs before theirs, yet again.
There is no earthly reason for this delay, except his own selfishness and manipulation in thinking that the nearer it gets to Christmas Day, the more you might want to back down so that their Christmas isn't spoilt. In the next couple of days, I expect you'll be hearing pleas from him along those lines, about it being too near the big day.
I'm sure you're one step ahead of him now. I'm just curious though, why did you agree to Weds? What excuse did he give for not coming tomorrow or Tuesday?
And can you confirm that you are going to give him 5 minutes to tell the truth and then pull the rug out from under him in front of the DCs, if he doesn't?