The Emperor Aurelian (r. 270 - 275) is credited with literally saving the Roman Empire. Aurelian became emperor at the lowest point of what historians call “The Crisis of the Third Century”.
After Emperor Valerian was captured, and the Roman army destroyed by the Sassanid Persians in 260, large sections of the Empire broke away from Rome.
In the east, the Syrians of Palmyra raised their own troops to defend themselves from the Sassanids, then began taking control of the Middle East and Egypt. In the West, the Roman general Posthumus declared himself the Gallic Emperor. This Gallic Empire comprised all provinces west of the Rhine.
It was Aurelian who re-conquered these areas, earning the name “Restitutor Orbis” (“restorer of the World”) from the Roman Senate.With such a mandate, Aurelian began a policy to rooting out corruption and reforming the debased Roman currency. He also introduced the worship of Sol Invictus (Invincible Sun) to Rome in 274. He credited this sun god with the victories he had gained against the Palmyrenes in the East. His plan was to allow Romans to acknowledge their other gods, but only worship Sol Invictus. In this way, he hoped to give the Empire a single common religion -- one God, one Empire, one Emperor.
The birthday of Sol Invictus was the winter solstice, which fell on December 25 on the Julian calendar. This was a day of celebration, drinking, and giving gifts to the poor. Early Christians thought the celebration of anyone’s birthday was an unholy, pagan practice, especially the birthday of a pagan god. Instead, they celebrated the anniversaries of the deaths of martyrs, honoring those who had paid the ultimate sacrifice for their faith. However, church fathers found it very difficult to rein in their flocks on the celebration of Sol Invictus. Like many other instances where Christianity conflicted with pagan rites, Christian leaders eventually transformed these holidays into Christian ones. December 25 was no different.
Early Jewish custom dated the sixth day of Creation, when God made Adam, Eve, and all the creatures of the world, as March 25. Early Christian scholars adopted that day as the date of the Annunciation, when the archangel Gabriel appeared before Mary to tell her that she would give birth to the Messiah. It was logical to assume that Jesus was conceived at that time. And what was nine months after March 25? December 25.
The first written source declaring December 25 as the birth date of Jesus is the Chronography of AD 354.In this book, a note for December 25 ( VIII Kal. Jan. in the Roman calendar) in the year 336 is annotated natus Christus in Betleem Iudeae ("Christ was born in Bethlehem of Judea"). This is the earliest record of Jesus’ birth being celebrated on December 25. From the tone of the inscription, it was a regular Christian holiday by this time, only 60-odd years since Aurelian introduced the cult of Sol Invictus.
So, in a very real sense, the pagan emperor Aurelian set forth the idea of a celebration on December 25 that involved the giving of gifts and celebration of a “holy light”, that was taken over by the worshipers of the birth of the Light of Jesus.