The season is now considered a Christian festival celebrating the birth of Jesus Christ but that is not how it started out. The English term Christmas ('mass on Christ’s day') is of fairly recent origin. The earlier term Yule may have derived from the Germanic Jōl or the Anglo-Saxon Geōl, which referred to the feast of the winter solstice.
Christmas originally had nothing to do with Christianity and pre-dates Jesus Christ by hundreds of years. It is based on pagan fertility rites and practices. Many of our Christmas traditions like decorating trees, singing carols and giving presents, are rooted in the traditions of non-Christian festivals.
We don't observe Christmas on 25 December because it was Jesus's birthday; no one really knows when he was born anyway. It's more because this was the date the Romans historically celebrated the (northern hemisphere) winter solstice. For each hemisphere, the winter solstice is the day with the shortest period of daylight and longest night of the year, when the sun is at its lowest daily maximum elevation in the sky. The fact that Christmas was celebrated on the birthday of the unconquered sun (dies solis invicti nati) gave the season a solar background, connected with the kalends, a Roman term for the first day of a month. On 1 January, the Roman New Year, houses were decorated with greenery and lights, and presents were given to children and the poor.