Christmas or Christmas Day is an annual commemoration of the birth of Jesus Christ, celebrated generally on December 25 as a religious and cultural holiday by billions of people around the world. A feast central to the Christian liturgical year, it closes the Advent season and initiates the twelve days of Christmastide. Christmas is a civil holiday in many of the world's nations, is celebrated by an increasing number of non-Christians, and is an integral part of the Christmas and holiday season.The precise day of Jesus’ birth, which historians place between 7 and 2 BC, is unknown. In the early-to-mid 4th century, the Western Christian Church first placed Christmas on December 25, a date later adopted also in the East. Theories advanced to explain that choice include that it falls exactly nine months after the Christian celebration of the conception of Jesus, or that it was selected to coincide with either the date of the Roman winter solstice or of some ancient winter festival.The original date of the celebration in Eastern Christianity was January 6, in connection with Epiphany, and that is still the date of the celebration for the Armenian Apostolic Church and in Armenia, where it is a public holiday. As of 2011, there is a difference of 13 days between the Julian calendar and the more generally used Gregorian calendar. Those who use the Julian calendar or its equivalents thus celebrate December 25 and January 6 on what for the majority of people is January 7 and January 19. For this reason, Ethiopia celebrates Christmas, both as a Christian feast and as a public holiday on what in the Gregorian calendar is January 7.