Christmas is an annual festival commemorating the birth of Jesus Christ, observed primarily on December 25 as a religious and cultural celebration among billions of people around the world.

Christmas Day is a public holiday in many countries,[13][14][15] is celebrated religiously by a majority of Christians,[16] as well as culturally by many non-Christians,[1][17] and forms an integral part of the holiday season organized around it.

The traditional Christmas narrative, the Nativity of Jesus, delineated in the New Testament says that Jesus was born in Bethlehem, in accordance with messianic prophecies. When Joseph and Mary arrived in the city, the inn had no room and so they were offered a stable where the Christ Child was soon born, with angels proclaiming this news to shepherds who then spread the word.

Although the month and date of Jesus' birth are unknown, the church in the early fourth century fixed the date as December 25. This corresponds to the date of the winter solstice on the Roman calendar. It is exactly nine months after Annunciation on March 25, also the date of the spring equinox. Most Christians celebrate on December 25 in the Gregorian calendar, which has been adopted almost universally in the civil calendars used in countries throughout the world. However, part of the Eastern Christian Churches celebrate Christmas on December 25 of the older Julian calendar, which currently corresponds to January 7 in the Gregorian calendar. For Christians, believing that God came into the world in the form of man to atone for the sins of humanity, rather than knowing Jesus' exact birth date, is considered to be the primary purpose in celebrating Christmas.

Source: Wikipedia