Merry Christmas?

Yes, it is both grammatical and OK. In fact many Christians prefer it, on the grounds that Christians should celebrate the aniversary of Christ's birth happily, rather than by just getting merry (i.e. eating and drinking and being jolly).

I'd like to say "Have A Warm Christmas Night!" Hey, I have a small present for all of you! A picture of Christmas Angel! Hope the angel will bring happiness and luckiness to you, your family and your friends! Have A Warm Christmas Night!