2. Don't shoot yourself in the foot.
Regardless of how thoroughly your sweetie may have debriefed you, there may well be a bit of wackiness that got left out of the 411. Also, people are gloriously inconsistent--the poshest Brahmins might break out truly hideous Rudolph sweaters on Christmas day; the most resolutely secular families might still have a longstanding cultural tradition of midnight mass. So don't snark on any holiday practice until you are positively sure that your sweetie's family doesn't participate in it: the kind of sureness only several years of joint holiday festivities can give you.