I think Norm is along the right lines. The X in Xmas is an from an old language, i could remember which one so Norm probably has it. So it's not an off the wall abbreviation. however, what kind of knob pronounces it Xmas??? it should always be pronounced Christmas.
Unless your Birmingham City Council, who once tried to change it to "winterval" because of the large muslim community. Funnily enough, the muslim community didn't like it, so they changed it back
idiots.
personally, i always write Christmas.