I say American Atheist because, well, it's a bit late for our friends in Canada, who don't have as big an issue with this as I understand. :)
For us down here in the US of A, though, the first of our real family holidays show up. Yes, we had halloween, but we don't typically have people joining together for halloween dinner or the like - it's more a time to let the kids dress up (or go to a party yourself.) So the first one - Thanksgiving - is on its way.
Thanksgiving is the first one that can be awkward for an atheist. Especially a new one. I don't know about your home or family, but mine had everyone together (or everyone local, depending) and giving thanks meant praying. It's not a religious holiday - though I'm sure some will paint it as one. But it still can lead to this awkward moment where you're asked to give thanks. Or the fact you're an atheist may come up.
If this is when you expect to out yourself... I'd generally suggest another time. It's easier, to me, to approach people one or two at a time when you think they can deal with the news and say "Hey, by the way" than stand up in the middle of everyone and go "Guess what, folks!" You don't want to be that person remembered for "ruining" thanksgiving. (Which some may see it as.)
You might mention it to one or two people at this time, though - that's up to you.
So what if it happens? What if you're all sitting at the table and they look at you and say "Hey, Phil (or Susan or Eddie or Jasmine or whatever your name is,) would you say grace?" Some of us are going to have *that* moment where we ask ourselves just what we do?
You can say no, of course, but that's awkward. You'll be "that guy," again.
Or you can think about it ahead of time, just in case, and when they ask you to give thanks, say sure - and then *give thanks.* This doesn't mean praying! This doesn't mean giving up who you are and what you believe (or acting like you believe in something you don't.) Remember, thanksgiving is not a religious holiday, or even act. Just *give thanks.*
"Instead of praying, I'd like everyone to just think about the last year. This day, today, that we're together is called thanksgiving - and we have a lot to be thankful for. That we're here at all to be thanksful, and to spend time with each other. That we have a roof over our heads and heat (or air conditioning, depending on where you are.) That we are fortunate enough to live in a country where we aren't fearful to leave our homes, we're not wracked by civil war or invasion. That we didn't have to go out day in and day out to farm the fields and raise the food we're gathered to eat tonight - someone did, and we should be thanking them, and the people who delivered and thanked it, but that despite recent times we're prosperous enough to be able to go to the store and buy the fruit of their labor...."
And so forth and so on. Just get people to think about what they *are* thankful for. Go with whatever your situation is. There's no need to mention a god at all.
And if you're not the one saying it? Sure, hold hands... they're your family. But keep your eyes open. Look around.
... Then get ready for hannukah or christmas.