I always say "Merry Christmas" to my Christian friends, "Happy Hanukkah" to my Jewish friends, and "Io, Saturnalia" to my pagan friends.
When I didn't know the beliefs of the person, I used to just say "Have a happy holiday."
But the Christians would read me the riot act since everyone knows they're the only religion in the United States that observes a holiday in December.
Now I just say Merry Christmas to everyone. The Jews, Pagans, and atheists don't go ballistic on me. They're not offended in the least. They ignore the error and merely appreciate the sincerity of the sentiment.
And I don't have to worry about getting a lecture from the Christians.
During this season of universal peace and good will, I just don't need the hassle.