Christmas Eve refers to the evening preceding Christmas Day, a widely celebrated festival commemorating the birth of Jesus of Nazareth that takes place on December 25. It is a culturally significant celebration for most of the Western world and is widely observed as a full or partial holiday in anticipation of Christmas Day.
One reason celebrations occur on Christmas Eve is because the traditional Christian liturgical day starts at sunset, an inheritance from Jewish tradition, which in turn is based in the story of creation in Genesis: "And there was evening, and there was morning – the first day." This liturgical day is followed for all days in the Eastern rite and the custom of beginning Christmas celebration (as well as Sunday and the other major festivals) in the preceding evening is preserved in western Churches that have altered the liturgical day to start at midnight, for example the Roman Catholic Church. Many churches still ring their church bells and hold prayers in the evening before holidays; for example the Nordic Lutheran churches. In some languages, such as the Scandinavian, Christmas Eve is simply referred to as "Christmas Evening".
(... from Wikipedia on 2012-04-17 11:50:20)