The Roman Catholic Church is responsible for establishing the date of birth of Jesus was December 25th, based on Pagan Religions from which the Catholic Church had it's beginnings.
In addition, people tied the Birth of Christ to the winter solstice, which came from the Roman God Saturn. Saturnalia. The date was originally December 21st but was tied to December 25th, the birth of the sun-god named Phyrgia, a culture in the ancient Balkans.
The Roman Catholic Church was unable to get rid of Saturnalia ; so early in the 4th Century, they adopted the holiday and tried to make it a Christian celebration of the Lord's Birth. They called it the Feast of the Nativity. This custom has been a part of western culture ever since.