And why the fuck couldn't the MIL book her party for the following weekend? If he DID tell his mum, rather than deliberately mislead the OP by telling her it was OK to book without checking first, then the MIL is doing it on purpose to try to 'make a point' that she MUST always come first to her son. And if that IS the case, then I damn well WOULD go on the trip on my own, rather than play those silly little games with the MIL. If that makes it hard for poor ickle husband, fuck it. Maybe he can grow some fucking balls to stand up to his wannabe matriarch mother.
And if he didn't check with his mother that the date was ok (she possibly could have booked for the following weekend, had she done it 6 months ago, or maybe it was the only convenient time for her DM to babysit?), then he deliberately misled the OP, telling her that it was OK when he didn't even KNOW. And then, to compound that, he then DEMANDS the OP's presence at the meal, knowing what an effort and expense she had gone to. In which case, if the OP going on this trip with a friend makes it hard for poor ickle husband, fuck it. Maybe he can grow the fuck up and actually CHECK arrangements properly when he is asked to...
Either way, it boils down to the fact that the OP should go on this trip, enjoy herself, and ignore all the shitty family politics that seems to plague her husband's family. At least SHE gets a break from her DC, and a lively trip out of it.
What's the alternative, OP? Going to a family meal, sitting there thinking "oh, we'd be in the theatre now", "oh, we'd be in the restaurant now", all the while getting more pissed off and resentful? Fuck that. You went to so much effort and expense, ENJOY that effort and expense. I certainly wouldn't be putting myself through that for the sake of either a husband who deliberately misled me AND couldn't be grown up enough afterwards OR a manipulative MIL, whichever it is.
He needs to tell you whether he actually DID check whether that weekend was OK with your MIL, so you know what the situation is.
Because if he DID check, and she booked OT then anyway, I'd be bloody FUMING, and certainly wouldn't be joining in a celebration on her behalf. I'd be too busy enjoying myself at the theatre.
However, if it was down to her husband NOT checking the date, when he told her he had, then it is up to him to make his excuses for his wife's absence, and explain to his mother what he had done, and why she wasn't there. And if I was the mother in that situation, I'd be texting his wife, telling her to have a great time and raise a glass to me, and not to worry as her daft as a brush husband has got a flea in his ear from me...