The urban legend that many Christians think is true is that early Christians appropriated the pagan, Roman Feast Of Saturnalia of Dec. 25 and converted it into the Christian holy day.
Nope. Saturnalia was not established by the Roman emperor until AD 274, when the date of Jesus' birth had already been set (though not widely celebrated) for three-quarter century.
It is the Western church that celebrates Christmas on Dec. 25. The Eastern church does so on Jan. 6, the date of Epiphany in the West. But curiously, the reason the West celebrates Christmas on Dec. 25 is the same reason the East does so on Jan. 6.
And that reason is (most likely) actually the dating of Easter, not Saturnalia.
Read it all here: "How December 25 Became Christmas," by Andrew McGowan.