Christmas is a Christian holiday held on December 25 which celebrates the birth of Jesus Christ. Eastern Orthodox Churches, which use the Julian Calendar to determine feast days, celebrate on January 7 by the Gregorian Calendar. Both dates are merely traditional and neither is thought to be the actual birthdate of Jesus. Christ's birth, or nativity, is said to fulfill Old Testament prophecies concerning the coming of a messiah, or savior. 　　The word Christmas is derived Middle English Christemasse and from Old English Cristes m?sse.It is a contraction meaning "Christ's mass". The name of the holiday is often shortened to Xmas because Roman letter "X" resembles the Greek letter Χ (chi), an abbreviation for Christ. 　　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.