The word for Christmas in Chinese is Shèng（圣） dàn（诞）or Shèng（圣） dàn（诞） jié（节）where jié （节） means festival. Merry Christmas in Chinese is said Shèng（圣） dàn（诞）kuài（快）lè（乐）or Shèng（圣） dàn（诞） jié（节） kuài（快）lè（乐）where kuài（快）lè（乐） means merry or happy.
So since its Christmas time， you can wish your Chinese friends a merry Christmas by saying 圣诞快乐（shèngdàn kuàilè）or 圣诞节快乐（shèng dàn jié kuài lè）. Chinese are always amazed when they see a foreigner who can speak Chinese, therefore, I believe your Chinese friends will be very happy to receive your message.
How to respond to 圣诞节快乐？
You can respond by simply saying xiè（谢）xiè（谢）. If you want to return the wish, you can add nǐ（你） yě（也）shì（是）which means wish you the same.
You can also reply 谢谢（xièxiè）, Wǒ（我） yě（也） zhù（祝） nǐ （你）shèng（圣）dàn（诞）jié （节）kuài（快）lè（乐）which means thank you, I also wish you merry Christmas.
Christmas in China
Christmas is not a public holiday in China, however you can expect to see Christmas-related items like Christmas trees and Christmas decorations in large shopping malls.
You can also expect to buy things at discount on online shopping platforms like táobǎo （淘宝）and jīngdōng（京东）. If you want to have a look at táobǎo, click here. If you want to have a look at jīngdōng, click here.
Words related to Christmas
shèng（圣）dàn（诞） lǎo（老）rén（人）which means Santa Claus.
shèng（圣）dàn（诞）shù（树）for Christmas tree.
shèng（圣）dàn（诞）kǎ（卡）for Christmas card.
This is the end of this article about Christmas in China, if you have anything to say about it, feel free to leave a comment. Wish you all a merry Christmas.