Remember my stories from Bishop's, of scatterbrained chamber ensemble colleagues who'd blow off rehearsals for "emergencies" such as being hungover, or needing to buy groceries, and of the time Flaky claimed that he didn't need me for chamber orchestra, only to pull a clarinet part out of thin air two weeks before the final performance, and then shanghaii me into playing it? I also have enough King Fluteloop stories to write an epic novel, lol. But anyway, my point is, I remember the advice that people on here gave me--they told me that "lack of planning on Flaky's/King Fluteloop's part did not contitute an emergency on mine." So, I think that that would be a good category for the Etiquette School forum, along with its abbreviated cousin, "Bed, made, lie." It's sort of an "extra-credit" version of "I'm afraid that won't be possible," or "No is a complete sentence." For example, here's a situation in which it might work:
OP (soliloquy): Yay, Thanksgiving is coming, I'm going to watch football with my husband, and then we're going to order Chinese food. After that, we're going to go for a walk outside to look at all the beautiful fall colours.
OP's MIL: OP, we're going to have Thanksgiving at YOUR house this year, which means YOU have to make a turkey dinner for thirty people, because I'm too OOOLLLLLLLD to do it anymore!!! You'd better get started too, because Thanksgiving is next week. Now, don't forget about Dia-Betty, Vegan Vernon, Lactose Intolerant Larry, Picky Priscilla, Fad Diet Fabio, and Carbless Carly!!!
OP: I'm afraid that won't be possible, my husband and I have already made plans for Thanksgiving.
MIL: But it's FAAAAMILY!!!! And I'm too OLLLLLD!!!! And I've already INVIIIITED everyone, so what am I supposed to DOOOOO?!??!?!
OP: I suggest you take the whole brood to Denny's, because lack of planning on your part does not constitute an emergency on mine.
MIL: You're so MEEEAAAAN, I'm going to cut you out of my will, and you can't come to my birthday party either!!! I'm going to tell all my friends that you have COOTIES, so they won't be your friends anymore!!!
OP: Well, then, I couldn't expect all those people to eat cootie-laden turkey, could I? Now, if you'll excuse me, I have to go exercise my channel-surfing thumb.