Some Biblical scholars place the date of Jesus' birth on this day.
In the modern west, Christmas is the celebration of Jesus' birth, but there's no reason to think that he would have been born then — especially since so many features of the nativity stories (such as shepherds) are inconsistent with a winter birth. A spring or summer birth is far more plausible.
Scholarly dating for the time of Jesus' birth varies quite a bit, with the earliest being in 7 BCE and the latest being in 2 BCE. Some scholars assume a narrower range of possible dates, 6-4 BCE.