Assuming you are no longer living a lie and pretending to believe, and assuming you've already come out to them as an unbeliever and it's not news, and assuming you even want to spend the holidays with them (rather than more liberal or godless friends), just don't bring up politics or religion. If you've chosen to be there, then there is no reason to challenge their beliefs on such a day, nor any reason to complain about their religious behaviors, which they are entitled to. Whether you're their guest or their host, courtesy demands that you be tolerant and diplomatic.
Be reasonable. For example, you can bow your head in silence at a table prayer. But if they ask you to give the blessing, politely decline. If they insist, politely warn them it will be secular, in accordance with your own beliefs. If they still insist, issue a secular thanks to all those actually responsible for the feast and the safety and prosperity that made eating it that day possible. Similarly, if anyone proselytizes or unreasonably makes you uncomfortable, politely remind them that they are being impolite. And if they start bringing up religion or politics, remind them that they might want to change the subject, as they will object to your opinions on the subject and it could get heated.
But if they press the issue, you warned them: tell them what you think and why, with a level head, polite demeanor, and abundant attention to facts and logic. If they get angry, remind them that it's impolite, and that they were warned they might not like what you had to say, and perhaps they'd like to change the subject after all.