The origins of the holiday and its December date lie in the ancient Greco-Roman world, as commemorations probably began sometime in the 2nd century. There are at least three possible origins for the December date. The Roman Christian historian Sextus Julius Africanus dated Jesus’ conception to March 25 (the same date upon which he held that the world was created), which, after nine months in his mother’s womb, would result in a December 25 birth.
In the 3rd century, the Roman Empire, which at the time had not adopted Christianity, celebrated the rebirth of the Unconquered Sun (Sol Invictus) on December 25th. This holiday not only marked the return of longer days after the winter solstice but also followed the popular Roman festival called the Saturnalia (during which people feasted and exchanged gifts). It was also the birthday of the Indo-European deity Mithra, a god of light and loyalty whose cult was at the time growing popular among Roman soldiers.
The church in Rome began formally celebrating Christmas on December 25 in 336, during the reign of the emperor Constantine. As Constantine had made Christianity the effective religion of the empire, some have speculated that choosing this date had the political motive of weakening the established pagan celebrations. The date was not widely accepted in the Eastern Empire, where January 6 had been favored, for another half-century, and Christmas did not become a major Christian festival until the 9th century.
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