Outing During Thanksgiving Dinner

I can't think of any circumstances that would make me out somebody (at least, not intentionally). If someone wants to be gay and in the closet it's their business.

It doesn't sound like the best settings for such an announcement. Holiday gatherings are usually packed with emotion as it is, and people might feel cornered and their reaction will be extreme and maybe not even according to their real emotions. If you have an announcement, I suggest saving it for each person at a time. Allow people to digest before forcing them to share the news with everybody else.

Having two sisters who are lesbians I can tell you this, it is something that should be discussed with some intimacy and low key surroundings with family, friends and those you love.  If no one has a clue that you or anyone else is gay the last thing a family wants to hear is "pass the gravy, and by the way I'm gay".  There is a time and a place for coming out yourself, I can't imagine there is one for outing someone else, as that is their decision and not yours.