How do you say Merry Christmas in Korean?

메리 크리스마스!

Merry Christmas!

Mae Lee  k lee smas 

NOTE: I wrote (Mae Lee K LEE SMAS) because there is no R sound in Korean language.

OR..

즐거운 성탄절 보내세요!

Have a ENJOYFUL CHRISTMAS SEASON!

Julguhwoon Sungtanjeol bonaesaeyo.

People usually say Merry Christmas (Mae Lee K Lee Smas to be precise). 

If you want to be a bit formal, you can say 즐거운 성탄절 보내세요 (Julguhwoon Sungtanjeol bonaesaeyo).
It means Have a enjoyful Christmas season. 성탄절 means 聖誕節 (a season a holy one borns).

In South Korea, Christmas is an official holiday. South Korean people also celebrate Buddha's birthday (석가탄신일) and Korea day (개천절), the day when Dangun (단군), a first Korean King ever recorded in history claims the birth of Korea as an nation. Korean Buddhist people celebrates Buddha's birthday and Korea Day (개천절) is celebrated as a religious holiday by Chundogyo (천도교) people. Chundogyo is believers of Dangun (단군).