It's a complete mix. Some wear just regular clothes, not even red or green, just shorts/t-shirts. Some wear regular clothes, but put one of those toy strands of Xmas lights around their neck. Some wear a Xmas themed shirt of some kind. Some go all out in full Xmas attire. It's awesome!
For me, I'm going to wear a Christmas Vacation movie themed t-shirt and probably a Santa hat of some kind.