Origins of the Religious Festival

As the Christian religion was establishing itself in the first centuries A.D., Christmas as such did not exist: the Church knew only the festival of the Resurrection which was solemnly celebrated at Easter. The first celebrations of the birth of Jesus "Natale Christi" appeared during the IVth century in the West when Christianity became the majority religion. The objectives were to Christianize the festivals of popular culture and pre-Christian cults which were celebrated during the month of December. In particular, Jesus as "Light of the World" or "Sun of Justice" was a substitute for the Mithraic Sol Invictus. The liturgy or the interpretation of the scriptures would influence and determine how the scenes of the life of Christ would be represented and particularly that of the Nativity. .
In 354, Pope Liberus instituted the Nativity on December 25 while the Eastern Church celebrated the birth and baptism of Christ on January 6 on the feast of Epiphany.
The choice of the date of December 25 was based ultimately on the desire of the Church Fathers to turn the people of the Roman Empire away from idolatry. From the Vth century, the festival of the Nativity took on such importance in the Christian world that it signalled the beginning of the liturgical year. This continued up until the XIth century when the period of Advent was added to the Christmas cycle and the first Sunday in Advent from then on became the start of the new liturgical year.