Christmas Eve is the day that, according to modern calendars, contains the evening, or first evening, of Christmas, a widely celebrated festival commemorating the birth of Jesus of Nazareth. 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.
The reason why Christmas starts on Christmas Eve is that the traditional Christian liturgical day starts at sunset, an inheritance from Jewish tradition and 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".
Since Christian tradition holds that Jesus was born at night (based in Luke 2:6-8) Midnight mass is celebrated on Christmas Eve, traditionally at midnight, in memory of his birth. The idea of Jesus being born at night is reflected in the fact that Christmas Eve is referred to as "Heiliger Nacht" ("Holy Night") in German, "Nochebuena" ("the Good Night") in Spanish and similarly in other expressions of Christmas spirituality, such as the song "Silent Night, Holy Night".
Non-religious people, or people who are not formal with definitions, may see the whole day as a day of celebration or as just the day before Christmas. Many people all over the world celebrate Christmas without being religious or even of a Christian background. The emphasis of celebration on Christmas Eve or Christmas Day varies from country to country and region to region (see below).
In Western culture, Christmas Eve is mostly celebrated on December 24. However, the Coptic, Serbian, Russian, Macedonian, Georgian, and Ukrainian Orthodox Churches, as well as the Greek Orthodox Church of Jerusalem, use the Julian calendar, which is currently 13 days behind the Gregorian calendar, so Christmas Eve for the adherents of those Churches coincides with January 6 of the following year in the Gregorian calendar.
So gimme my presents!!