It is not easy to pin-point the origins of the Christmas feast, today the more important feast of the Christmas season in most western Christian churches. One can only say for certain that the birth of Jesus Christ was being celebrated in Rome by the year 336 A.D.; afterwards the feast was celebrated in other Christian churches throughout the world.
Why it was celebrated on December 25th is another question. No date for the birth of Jesus can be found in the New Testament, which is concerned more with the question "Who is Jesus?" than the date of his birth. Early Christian speculation about his birth date was influenced by the symbolism of the changing seasons, then popular in religious thought, which paid careful attention to the equinoxes and solstices of the sun. Christian scholars speculated that Jesus was conceived at the spring equinox (March 25th) and therefore was born on December 25th, the date of the winter solstice.
In many of the Christian churches, March 25th is still the Feast of the Annunciation, when the Angel Gabriel announced to Mary that she was to be the mother of Jesus.
Possible impluse for the feast of Christmas may have came too from the establishment of the pagan feast of the "Unconquered Sun-God" by the Emperor Aurelian in 274 A.D. to be celebrated on December 25, the day of the winter solstice in Rome and throughout the empire. In response, Christians could celebrate the feast of the "Sun of righteousness" (Malachi 4, 2), Jesus Christ, who called himself " the light of the world." (John 8,12 )