"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.

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." grammarist.com