It's possible that:
* It's a typo. Maybe the intended phrase was "Pocket Christmas" or something similar.
* It's a playful or creative invention. Perhaps someone created it for a specific purpose or as a humorous phrase.
* It's a language from a fictional world. Could it be from a book, game, or movie?
Without more context, it's difficult to determine the meaning or origin of "Poket Kristmet."