(See also: red shit: a shit that makes your butthole bleed; Green shit: speaks for itself. green shit.) A Christmas Shit is basically a combination of the Red and Green Shit. You see both red and green. The Christmas Shit also earns its name due to the fact that it often takes the shitter by surprise, as if it were a present.
I just took a Christmas shit in July and I didn't even know it was coming!
When a group of people stay up until midnight on Christmas eve, then all take a dump together in an outdoor setting
Sis: We're going to be up late tonight. We have to take our Christmas Shit in the back yard.
Bro: oh right, I almost forgot about that.
The long amounts of time spent on the toilet after a heavy Christmas dinner.
Person 1: How was your Christmas?
Person 2: Good, except for the Christmas shits I had.