Causehead is an idiot, and is way out of line in making her beliefs center stage here. That said,
Eff you guys! I am so hungry for a traditional homemade turkey dinner now, it’s actually painful. 
“As God is my witness, I thought turkeys could fly.” 
Seriously, another vote for “the host sets the rules, and if the guests don’t like them, they don’t have to come.”
True. Honestly, Thanksgiving has become better for me since I became vegetarian. Do you know how many veggie-friendly side-dishes are served at your typical Thanksgiving meal? I get stuffed on those alone, thank god I’m no longer trying to squeeze some turkey and gravy in there too. (I’m surprised that meat eaters don’t just asplode from it all… )
I wouldn’t say anything at this point. The invitation went out. Done. Type up your “missed you at the family gathering” email and save it as a draft.
If vegan-diva’s SO calls and asks for an update you can truthfully say everybody hopes he still comes and they all understand that a vegan wouldn’t be interested in a turkeycentric dinner.
When we were cooking for my vegan sister in law, we discovered that quantity was near zero.
Stuffing was right out, along with the turkey.
The margarine we used as a household wasn’t vegan, it was made with milk solids or something. So all the veggies, even when we took on the butter and put in margarine, were a no go.
Our traditional salad had ranch dressing. Yeah…but that was what everyone liked.
Our yams are topped with marshmallows - which are, of course, not vegan (gelatin).
And pumpkin pie has condensed milk and eggs in it.
Every recipe I normally make for Thanksgiving had to be modified, INCLUDING the cranberries - no refined sugar.
Vegetarians are generally easy to feed - although the gelatin thing would throw me for years - I’d just forget. Vegans, particularly crabby, difficult to please, “if you cared about me you’d throw out all your family traditions and make a nice vegetarian lasagna with soy cheese” vegans are a pain in the ass for holidays that have “traditional” recipes.
(She couldn’t eat any Christmas cookies either - eggs.)
So… the rest of you guys lead lives of leisure where you can just have Thanksgiving whenever you want to? Because I get Thursday off, period.
I can’t speak for anyone else, but for us we get together on whatever day the most people can make it, and anyone else tries to make it when they can (after work, a short amount of time before work, etc). For my SO’s family, that frequently isn’t Thursday. Two years in a row we’ve done Thanksgiving dinner three nights, which is exhausting, but includes all of our loved ones.
I have so many ideas about how to address this.
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Invite Causehead to a fake dinner, and the rest of the family to the REAL dinner, like the fake prom they made up for the gay high school girl in Mississippi. Oh yeah, don’t be home at fake dinner time.
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Explain that the dinner will include a Satanic ritual designed to commit the souls of the pilgrims to the inner circles of hell. It’s all about payback for the natives.
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Distribute a poll to your family, asking whether Causehead should expelled from the family for being such a douche.
I wonder if there’s anything significant about who assumed Causehead was male, and who assumed Causehead was female. 
I’m a little too lazy right now to find where the OP specified, if at all, but I know it wasn’t IN the OP.
Sounds like it’s time to break out the turducken.
It depends on your job. In my family, my husband helps operate a big ecommerce site. He works Thanksgiving. And the day after. And pretty much seven days a week from mid-October to mid-January. He works from home on Thanksgiving, dials in - so its at my house. Christmas and Christmas Eve are usually a few phone calls he needs access for - laptop works. It used to be worse.
My sister and her husband are hospital employees. Sometimes they get Thanksgiving weekend off. Sometimes we move it. Same with Christmas.
I, however, work in a factory. We close down for four days. Then at Christmas time, we furlough for an entire week. I get lots of vacation, but about half the days are chosen for me as furlough.
Above all else, make sure that Vegan Drama Queen is staying at a hotel, then. Not at your place.
Because you’ll want some place to kick her out to, when the “meat is murder” drama ensues when you put the turkey on the table. Whether it’s Thursday or Friday.
It’s in one of the poll responses - “She’s right, you’re wrong. Thanksgiving is a misguided holiday.”
Wow, this thread has turned a bit savage. The woman is a diva, bitch, drama queen, foot stomper, idiot, jerk, and moron who deserves to be lied to, uninvited, asked to stay home, and ejected from the family because she is… rude.
May I remind everyone that all we know about this woman is two things she may or may not have said, and that we got fourth hand? For a board that claims to fight ignorance, we seem to have mighty flimsy standards for evidence.
Rumor_Watkins, I asked before and I’ll ask again – have you even met or spoken to this woman personally?
Even if the rest of the familiy is fine with moving the dinner, it’s not their call either. You are the host. You want to celebrate Thanksgiving on Thursday. That’s the dinner you’re serving. They all can come or not.
If Causehead wants to host another dinner on Friday, more power to her.
I have the perfect solution.
You’ll just have to cook and eat Causehead. The turkey survives, the family eats, and you don’t have to worry about her fucking up Thanksgiving next year.
Yeah, my prediction is that you’d either get this reaction or passive-aggressive sighs, eye-rolling, “ew” body language, etc.
But I agree you should talk to her one-on-one first. See how the conversation goes, how rigid or flexible she is towards other people’s eating, etc.
Then come back here and give us the gossip.
“It’s happening on Thursday, I hope you can join us.”
Aha! Didn’t even think of looking at the poll responses!
Now I’m really interested in those who assumed she was male. 
Under different circumstances, I may well agree with you, but this was a poll thread and asked for opinions based upon the information we were given. Based upon the information we have, the opinion that she is any of those things is valid inside those parameters. Give us more information, and some of us are open to changing our minds.