Of Festivals names: Why is Christmas called X-Mas?

Thinking of festivals, Christmas is unique that it has a short name - X-Mas. Probably because X men celebrate this festival too? Or because it kinda gets very cold in Northern Hemisphere on Christmas ;) 

Why X-Mas? No Other festival has an acronym! Deewali is not wished Happy D wali. Ram Navami was not shortened to Ram 9mi! Eid is always Eid and never Id.! Dushera was never 10era! Independence day is not I-Day! X-Mas is not SMS lingo because even Thanksgiving is never typed as Thanx Giving!

Why X-Mas then? X got something to do with cross? Cross-mas? But I think Cross would be more meaningful around the good Friday time-frame. Or are you telling Jesus: "Happy Birthday Jesus! Watch out for that Cross!!"

 Shortening festival names is interesting idea though. Makar Sankranti (Capri-Sank), Vaikunth Ekadashi (V-11), Krishna Janmashtami (K-Janm8mi), Guru Gobind Singh Jayanti (GGSJ) could be some real good ones to start with!