No, I don't mean to a sing-along screening of Frozen (although that actually sounds like fun). If you know that you feel overwhelmed in large crowds and tend to overthink interactions, then practice conversation starters. Not only will this help you feel more in control, but it could potentially be a lifeline for the other person. After all, festive celebrations are often a chance to meet people for the first time, e.g. your cousin's girlfriend or your boss's wife.
Top tip: maybe move beyond the obvious "What are you doing for Christmas?" and try something light and fun such as "What would be your dream present?"