Merry Christmas to you and yours! Instead of being repetitive, here’s a link to last year’s Christmas post.
“Merry Christmas” in Arabic is عَيد ميلاد مَجيد (ʿīd mīlād majīd), which more literally means “Glorious Christmas.”
Christmas is عيد ميلاد (ʿīd mīlād), but this raises the (slim) possibility of confusion as this is also the Arabic translation of “birthday”; literally, “celebration (ʿīd) of birth (mīlād).” So if you want to be specific, you could say عيد ميلاد مَسيح (ʿīd mīlād masīḥ) to specify “birthday of the Messiah” (which applies whether you’re talking to a Muslim or Christian, as the Quran acknowledges Jesus as the Jewish Messiah), or even go with الكريسمَس (al-krīsmas). On the other hand, if it’s December and you’re talking about ʿīd mīlād, I think you’ll be understood.
However you choose to say it, Happy Holidays to you and yours!