"Christmas cake" is quite an old expression. It was most popular in the 80's and was already falling out of use by the late 90's. On a Yahoo questions board in 2008, a Japanese user wondered what it means, and was given an answer that it is a dead slang. And yet we, the anons, keep using it to this day.
In a cosmic ironic kind of way, the word "christmas cake" itself has turned into a christmas cake, and we're the only ones willing to eat it in this day and age.