Christmas Adam is a colloquialism for the day before Christmas Eve or December 23rd. It is a rather new term. An online slang dictionary created a listing for it only nine years ago. Christmas Adam is not listed in any formal dictionaries and we had a hard time finding formal writing examples of its use.
The general consensus of the origin of the name is that in anticipation for Christmas to come, people were looking for a name for the day before Christmas Eve. Some were calling the 23rd Christmas Eve Eve. Then someone (or more likely several someones) realized that in Christian mythology God created Adam before he created Eve, so the day before Christmas Eve would be Christmas Adam.
It is hard to tell if this term has traveled outside of the United States, since it is too colloquial to be used in most writing. However, if you do intend to use it as a greeting, pair it with the appropriate adjective (e.g., Merry Christmas Adam in the United States and Happy Christmas Adam in other countries.)
We’re wondering what your family does that’s fun and, well, different. Does your Nativity scene include Army men or other unusual characters? Do you do something special on Christmas Adam (the day before Christmas Eve)? [BND Magazine]
One of my facebook friend’s status wished everyone a “Happy Christmas Adam!” today … am I the only one who’d never heard this phrase? [Still Thinking]
No letters, please, from the folks extolling the virtues of early shopping. I tried that once — if you consider Christmas Eve Eve early. [News Sentinel]