Xmas because Roman letter "X" resembles the Greek letter Χ (chi), an abbreviation for Christ (Χριστ??).
In Western countries, Christmas has become the most economically significant holiday of the year. The popularity of Christmas can be traced in part to its status as a winter festival. Many cultures have their most important holiday in winter because there is less agricultural work to do at this time. Examples of winter festivals that are believed by some to have influenced Christmas include the pre-Christian festivals of Yule and Saturnalia.
In Western culture, the holiday is characterized by the exchange of gifts among friends and family members, some of the gifts being attributed to Santa Claus (also known as Father Christmas, Saint Nicholas, and Father Frost). However, various local and regional Christmas traditions are still practiced, despite the widespread influence of American, British and Australian Christmas motifs disseminated by film, popular literature, television, and other media.