Natalis Solis Invicti, meaning "Birthday of the Unconquered Sun," was a festival celebrated in ancient Rome on December 25. It honored the sun god Sol and marked the winter solstice, symbolizing the return of longer days and the sun's rebirth. This festival was part of a broader tradition of sun worship in various cultures. The celebration included feasting, gift-giving, and various rituals to ensure the sun's strength. Over time, Natalis Solis Invicti influenced the choice of December 25 as the date for Christmas, blending pagan and Christian traditions in the Roman Empire.