Merry Christmas is Sheng Dan Kuai Le (Traditional: 聖誕快樂; Simplified: 圣诞快乐) in Chinese.
Sheng (聖) is sacred or holy. It is also short for sheng ren, meaning a sage or a saint.
Dan (誕) is birth or birthday.
Kuai le (快樂) is happiness, joy, delight, or rejoicings.
So, Sheng Dan Kuai Le, Holy Birth Happy, is how you say and write Merry Christmas in Chinese.
For Christians who believe Jesus is more than a saint, there is another common phrase for Merry Christmas in Chinese: Ye Dan Kuai Le (耶誕快樂).
Click Ye Dan Kuai Le to see what it means, how it is pronounced, and how it is written.