Holyday of the Nativity of our Lord and Savior Jesus Christ (which is how we name it in the Orthodox Church).
Veins start popping out when I think of someone telling me I am no longer allowed to say....
Merry Christmas.
LVM
The word derivation of "Christmas" (= "Christ" + "Mass" [as in Catholic]) is, I think, almost exclusive to the English language. It's ironic that the Romance languages themselves use a term which does not reference Catholic doctrine but rather is analogous to "nativity" ("Noel," "Navidad," etc.), whereas the Protestant Anglican church calls it "Christmas."
Merry Nativity (or whatever you choose to call it)!