(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!