Io Saturnalia! wrote:

The answer is really simple. So I'll keep this short.
Because there was no popular consensus among renowned
historians, astronomers, astrologers of the day, etc.,
with regard to the actual year, month, date, and time
of the Nativity of Jesus in Bethlehem; in 274 AD, the
reigning emperor Aurelian saw fit to unite the winter
solstice celebrations, most popularly Saturnalia, and
'Natalis Solis Invicti', "Birth of the Sun-Invincible"
with the growing Christian tradition of observing the
nativity of Jesus 'Natalis Filius Invicti', "Birth of
the Son-Invincible". There's a lot more to understand
about this, but I promised I'd keep it simple, to wit.
For information on historical calendar dates relating
to Jesus' birth, life, death, and resurrection, see:
http://www.freewebs.com/danieljosephmin/Mins_Historical_Calendar_Of_Jesus.pdf
(in print where available)

An addendum to this: Saturnalia was AT the equinox when the Julian
calendear was established. By the late 3rd century A.D., it was off 3
days. - A. McIntire