It's also when christmas is celebrated in the Orthodox church.
One tradition I read long ago is that water will turn to wine on that midnight. I wish I could remember where I read it; I was supposed to be studying something in college. The story also had a warning in it. A man hid in the barn to see if the animals talked, presumably to hear about the fortunes of the coming year, or perhaps he hoped they'd mention a hidden treasure. At midnight, two voices were heard in the barn. "The water is wine!" "Yes, and you're mine."
By the way, Richard Chase's book Grandfather Tales has some nice stuff about Old Christmas in western North Carolina, including a mummer's play.