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為什么要拼寫成Xmas？尤其是當與"sale"這個詞聯(lián)系起來

 　　根據(jù)書中記載,你也許會感到驚訝……,通過Jeff.Rovin,Christ一詞是從希臘文Xristos來的,Xmas是它的縮寫,16世紀開始在歐洲流行

　　Xmas一詞在廣告中很常用,很可能是因為Xmas與sale兩個詞有同樣的字母數(shù),而Xmas明顯的比Christmas短