For the food prep part, I'd wear clothes I don't mind spilling on. Then change about 30-60 minutes before the first guest is due to arrive, and put an apron on over my outfit.
If people come early, just say, "hey, I'm going to go get changed", and do so.
How fancy is this gathering? We're very casual in my family, so I'd probably wear skinny jeans or pants, a pretty, festive top, and some pretty flats that I can move around quickly and easily in, because you're going to do a lot of last-minute jumping up and down.
Stuff that's easy to move around in, and stuff that lets you eat to your heart's content without feeling restricted too much by clothing!
ETA: we usually do Thanksgiving in Florida, so I left out a festive topper, which I would do in a colder climate for sure.