I always used to be a little offended when i seen people use the word Xmas, I believed the the saying its no good because it was just another way to secularize the special time and take Christ out of Christmas. But I feel better about it now, as...
Wiki says,
Christmas is sometimes shortened to Xmas, an abbreviation that has a long history.[4] In early Greek versions of the New Testament, the letter Χ (chi), is the first letter of Christ (Χριστός). Since the mid-sixteenth century Χ, or the similar Roman letter X, was used as an abbreviation for Christ.