just as in english
메리 크리스마스 merry christmas
즐거운 크리쓰마쓰 (jeul-kkeo-un keu-ri-sseu-ma-sseu), it means "Joyful Christmas
and the direct translation of "Merry Christmas" is 즐거운 성탄 (jeul-kkeo-un seong-tan).
Well, Koreans don't say Happy Holidays or Seasons Greetings. That's so rare.
Pronunciations :
* 즐거운 (juhl-kkoh-uhn) 크리쓰마쓰 (kuh-rih-ssuh-mah-ssuh)
* 즐거운 (juhl-kkoh-uhn) 성탄 (soung-tahn)