Thoughts please? Dinner with friends..

Her loss, maybe it's a blessing in disguise she isn't going. If she was interested in any type of friendship she would not have booked a hotel with other friends for the same night, one less for you to worry about.

Sounds like she put her mates ahead of a group of common interest friends that she sees from time to time?

Honestly, I'd probably do the same, and I certainly wouldn't take it personally if someone did it to me. My old friends are more important than my casual acquaintances, that's just how it is.

While I understand why your nose is out of joint over this, I think you're making too big a deal over this. You've decided that this meet-up with old school friends is less important than your gathering. Maybe it is to you but you don't seem to know or care what her bond with these old school friends is like. There are people out there who have stayed in touch with their old schoolfriends and make the effort to catch up with them. For example, I have a friend in her late 40s who still meets up with a few of her old secondary school friends once a year or so. They get dressed up and book a table in a fancy restaurant and make a night of it.

My advice to you is to go ahead with your gathering and just present the others with the facts (i.e. Mary's not coming now, she's going to spend the weekend with some old schoolfriends). They'll all have their own opinions on what has happened. Be careful what you say because there's potential here for this to turn into a bitching session. Once that happens, you don't know what the fallout could be down the road. Your words could be repeated back to "Mary" or others might start seeing you in a different light. I agree that she seems to be flaky and you're not wrong to be annoyed. But this isn't a reason to fall out with her. Just work in the basis that she night let you down again in the future and concentrate on your other friends. By all means continue to invite her but don't go out of your way to accommodate her.

I'd say let the 5 of you.go ahead and enjoy your night.

Why rearrange it for one, especially one that did the picking in the first place and then finds a different thing to do.

Have you, as a group, allowed her to dictate events/dates in the past?

Not fair on everyone else imo.

I wonder will she expect you to rearrange now that she can't (/won't) make the night?

I agree with others, you come across as making quite a big deal of this in your own head (but it's too easy misinterpret things in writing, in the absence of whole context, so I won't go any further with that, I could be very wide of the mark) - but also that you should go ahead with the dinner with your other 4 friends, and enjoy it.

I do have a friend, one of a group of pals from a particular context, around whom everyone else in the group seems to pivot, so I can see where you're coming from in one sense - but equally, she shouldn't dictate (whether consciously or subconsciously) what the rest of ye do.

I would hate to see what you would do if she did something worse on you. Off the Xmas card list, never meeting her one to one. Maybe there’s history and this is the straw that broke the camels back but I do think if this is the first and only time she has ditched you then it is a weirdly obsessive over reaction. Even giving it as much thought as you have seems odd. I am a male though and maybe we’re wired differently. If it was a friend of mine I’d give him a bit of slagging on the next night out “oh are your real friends busy tonight” but other than that I’d just get back to normal after our 5 person night out.

Personally, I would be annoyed in your shoes. I have no time for flaky friends and I've distanced myself from a few over the years who were constantly cancelling plans last minute or being generally unreliable.