The word is Christmas and I won't use "Xmas." Why should I.

No should about it, BUT.....

Someone, of this parish, already pointed-out, and I like the sentiment:

X in Xmas is for the undetermined quantity. EG Hanukkah, Diwali, Ramadan, Samhain, and maybe Imbolc, Beltane and Lughnasadh.
It is a catchall for non-Christians.

Which is why I, tongue in cheek say Dawkinsmas, in lieu of any suitable appellation for Atheists.