The holiday party is so ubiquitous that everybody attends, especially if it's an office party. That includes guests who are Jewish, Muslim, Buddhist, Zoroastrian, Hindu, atheist, agnostic, you name it. Don't automatically assume that the person you just met is Christian and celebrates Christmas. Likewise, don't assume that someone is heterosexual, married or has children. Steer clear of such topics until they offer up enough personal details to guide you in your conversation.