I suppose it could mean both Christmas-based and just Christmas time in general, but my vote goes to Christmas Nights on the Sega Saturn
Originally, Nights was a flagship title for the Saturn, but they also released a stand-alone version called Christmas Nights just for...well, Christmas! Great little game.
If I remember correctly, once your memory system hit December 25th, something else happened in the game?
Can anyone beat this for the most Christmassy game ever?