Dies Natalis Solis Invicti

Christmas in Ritual and Tradition, Christian and Pagan by Clement A. Miles

The actual birth date of Jesus Christ (4 BC - 30 AD) is unknown, and Christians didn't have a Nativity feast until the 4th century. The earliest certain mention of December 25 as Christ's Birthday occurs in the Philocalian calendar (354).

At first, January 6, the Epiphany, was observed as the feast of Jesus's Baptism, with a secondary emphasis on His birth. The Armenian church still keeps Christmas on that day.

December 25 was already a major festival in the pagan Roman world, the Dies Natalis Solis Invicti, or " Birthday of the Unconquered Sun ", falling within the week-long celebration of the Saturnalia, a feast honoring the renewal of the sun at the winter solstice. Pagan celebrations on December 25 had included feasting, dancing, and giving gifts. So when this became a Christian festival, the customs continued, but with a Christian meaning imparted to them.

Did you know that Christmas become a legal holiday in America
not until the 19th century?

Source:

Miles, C.A., Christmas in Ritual and Tradition, Christian and Pagan (1912)

Encyclopedia International by Elizabeth M. Downie, Rosemary Hall.