Why is Christmas sometimes spelled Xmas?

      Why is Christmas sometimes spelled Xmas? Especially when combined with the word "sale"?

       According to the book Did you ever Wonder... by Jeff Rovin, the word for Christ in Greek is Xristos. The use of the shortened form "Xmas" became popular in Europe in the 1500s.

       The word Xmas is so common in advertising most likely because "Xmas" and "sale" have the same number of letters, and "Xmas" is significantly shorter than Christmas.

          Christmas为什么要拼写成Xmas？尤其是当与"sale"这个词联系起来

 　　根据书中记载,你也许会感到惊讶……,通过Jeff.Rovin,Christ一词是从希腊文Xristos来的,Xmas是它的缩写,16世纪开始在欧洲流行

　　Xmas一词在广告中很常用,很可能是因为Xmas与sale两个词有同样的字母数,而Xmas明显的比Christmas短