In the Roman Mithraic mysteries, December 25 was celebrated as the birthday of Sol Invictus, the Unconquered Sun, identified with Mithras. Roman soldiers across the empire gathered in underground mithraea to celebrate their god’s birth four days after the solstice, when the lengthening of days first became perceptible. The connection between the Zoroastrian Mithra and the Roman Mithras remains debated among scholars, but the transmission of Persian solar theology through the mystery cult profoundly shaped late Roman religion and its December festivals.