Christmas is also sometimes known as Xmas.

Some people don't think it's correct to call Christmas Xmas as that takes the Christ(Jesus) out of Christmas. Christmas有时候也被写为Xmas。有人觉得这样是不对的，因为把Christ去掉了。

But that is not quite right! In the Greek language and alphabet, the letter that looks like an X is the Greek letter chi / Χ which is the first letter of the Greek word for Christ.

但是这种写法（Xmas）其实是对的。

因为在希腊语和字母中，X是上帝耶稣基督这个词的第一个字母（X=Chi）

Its use dates back to the 16th century. 这种用法可以追溯到16世纪。