Personally, I would not leave it unsaid that you need to cancel if he is not going to the weekend. Just ask in a business-like way if is planning to go or not.
I would do that................and then depending on the what he replies and HOW he replies...........if you don't go to retro you could think of other things you could do without the children- motorbike rides, window shopping motorbikes ....whatever. However, if his reply is really negative to the Retro question, ( and I don't mean just about Retro.....rather about your whole sitch altogether.....then I would make plans for yourself - pamper yourself- no chores, bubblebath, movies, good book etc....but nothing for him or his children whilst the children are away.
BTW - stop torturing yourself with the FB stuff. FB is just a childish toy and it causes so many upsets and miscommunications. What IS the point in looking at it - why would he wipe off OW at this point? Doesn't mean they are in contact and he knows if you are checking up on him that this is going to hurt you - just stop. Another way he can stick the knife in without having to do anything.....and you are letting it work - {sigh}
Saffie me 46 H 46 M in 1986 D20,D18,S16,D13 H's A 01/05 to 07/06 H recommitted to M 07/06 renewed vows 09/06 Going from strength to strength