Christmas Eve is the day before Christmas Day, a widely celebrated holiday 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.
In Western culture, Christmas Eve is celebrated on December 24. However, the Coptic, Serbian, Russian, Macedonian, and Georgian 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. (From: Wikipedia, license: CCA-SA)