why do they say xmas instead of christmas

You're right, "xmas" is a common abbreviation for "Christmas." The "x" in "xmas" stands for the Greek letter "chi," which is the first letter of "Christos," the Greek word for "Christ." The use of "xmas" instead of "Christmas" has been around since at least the 16th century, and it's a way to show respect for the religious significance of the holiday while also recognizing the secular aspects of the holiday season.