Is it permissible for me to say “Merry Christmas” to my non-Muslim classmates, friends, family, neighbors, and others this holiday season?
Ayatollah Sistani’s Answer to Question on saying merry Christmas to Christian people:
Question: What is the ruling on saying assalaamu [alaikum] (Islamic salutation, meaning peace be with you) to the People of the Book or unbelievers? Also, is it permissible to send them seasonal greetings, such as on Christmas?
Answer: There is no harm in initiating the salutation, albeit makrooh (undesirable act) except out of necessity, under whose remit comes urf. Responding to their salutation should be by uttering [the word] alaik (with you). There is no harm in greeting them on their occasions.
Ayatollah Khamenei answer to question about celebrating Christmas:
Question: In common with Christians, some Muslims celebrate Christmas. Is there a problem in that?
Answer: There is no harm in celebrating the birthday of the Holy Jesus Christ (May peace be upon him and our Prophet and his pure progeny).