In katakana.  Translated into roman characters, that would come out as something like "Merii Kurisumasu."

Japan acknowledges Christmas, largely (I suspect) due to Disneyland (in fact, one of the most vital and sacred cultural centers in the country) Christmas Pageants that have built awareness of the foreign holiday year after year since the 1960's.

However, here it's largely an excuse for occasional christmas decorations, sales of fried chicken (Kentucky Fried Chicken is another Christmas promoter), sales of "Christmas Cakes" (no idea where they got that idea), young and female retail empolyees being forced to wear cute Santa costumes, and romantic dates.  Christmas is mostly celebrated as a romantic holiday for couples.  New Years serves in the  "traditional family gathering holiday" role here.