즐거운 성탄절 보내세요. 즐거운 jeul-geo-un *joyful, merry 성탄(절) seong-tan(-jeol) *christmas, directely means holy birthday 보내세요 bo-nae-se-yo *send please If you wanna say it to younger person or friend, 성탄(절) 잘 보내! 잘 보내! jal-bo-nae *send well! Or you can say just 메리 크리스마스 (Merry Christmas). Korean also use it a lot.

maybe 메리크리스마스 or 즐거운 성탄절!