Although the dating as December 25 predates pagan influence, the later development of Christmas as a festival includes elements of the Roman feast of the Saturnalia and the birthday of Mithra as described in the Roman cult of Mithraism.
The Chronography of 354 AD contains early evidence of the celebration on December 25 of a Christian liturgical feast of the birth of Jesus.
This was in Rome, while in Eastern Christianity the birth of Jesus was already celebrated in connection with the Epiphany on January 6.
A feast central to the Christian liturgical year, it is prepared for by the season of Advent or the Nativity Fast and initiates the season of Christmastide, which historically in the West lasts twelve days and culminates on Twelfth Night; and is an integral part of the holiday season, while some Christian groups reject the celebration.
In several countries, celebrating Christmas Eve on December 24 has the main focus rather than December 25, with gift-giving and sharing a traditional meal with the family.
While the month and date of Jesus' birth are unknown, by the early-to-mid 4th century, the Western Christian Church had placed Christmas on December 25, Today, most Christians celebrate Christmas on the date of December 25 in the Gregorian calendar, which is also the calendar in near-universal use in the secular world.
However, some Eastern churches celebrate Christmas on the December 25 of the older Julian calendar, which currently corresponds to January 7 in the Gregorian calendar, the day after the Western Christian Church celebrates the Epiphany.