Christmas, literally meaning, "Christ's Mass," hasn't always been celebrated at the end of the year.
In fact, no one knows for sure just when Jesus was born.
Scholars believe the confusion began in part because Christmas wasn't an early feast day in the church. And there was much disagreement over whether it should even be added to the church calendar.
But in Luke's version of the Gospel, we know that shepherds were outdoors, caring for their sheep at the time of Jesus' birth. Some have taken that to mean it must have been a warm time of year.
Early theologians celebrated the occasion on May 20. Others preferred late March or April, around the time of the Jewish Passover.
Dec. 25 falls during a time of year when pagans once celebrated winter festivals. It's believed the early church fathers picked the now-familiar date as an alternative to those ancient festivals.
Still, not everybody celebrates Christmas on the same day, even now.
Russian and Greek Orthodox churches continue to observe the old Julian calendar, celebrating Christmas on Jan. 7.