Gerard and Patricia Del Re explain the evolution of Dec. 25 becoming an official Roman celebration: "Saturnalia and the kalends [new moon] were the [pagan] celebrations most familiar to early Christians, December 17-24 and January 1-3, but the tradition of celebrating December 25 as Christ's birthday came to the Romans from Persia. Mithra, the Persian god of light and sacred contracts, was born out of a rock on December 25. Rome was famous for its flirtations with strange gods and cults, and in the third century  the unchristian emperor Aurelian established the festival of Dies Invicti Solis, the Day of the Invincible Sun, on December 25.
"Mithra was an embodiment of the sun, so this period of its rebirth was a major day in Mithraism, which had become Rome's latest official religion with the patronage of Aurelian. It is believed that the emperor Constantine adhered to Mithraism up to the time of his conversion to Christianity. He was probably instrumental in seeing that the major feast of his old religion was carried over to his new faith" (The Christmas Almanac, 1979, p. 17).
Although it is difficult to determine the first time anyone celebrated Dec. 25 as Christmas, historians are in general agreement that it was sometime during the fourth century.
This is an amazingly late date. Christmas was not observed in Rome, the capital of the empire, until about 300 years after Christ's death. Its origins cannot be traced back to either the teachings or practices of the earliest Christians. The introduction of Christmas represented a significant departure from "the faith which was once for all delivered to the saints" (Jude 3).
Excerpted from the booklet Holidays or Holy Days: Does It Matter Which Days We Observe?