"Prior to the
celebration of Christmas, December 25 in the Roman world
was the Natalis Solis Invicti, the Birthday of the Unconquerable
Sun. This feast, which took place just after the winter
solstice of the Julian calendar, was in honor of the Sun God,
Mithras."
-
Celebrations: The Complete Book of American Holidays
by Robert
J. Myers
"In the Roman
Empire, Mithra became associated with the sun, and was
referred to as the Sol Invictus, or unconquerable sun. The first day of the
week -- Sunday -- was devoted to prayer to him. Mithraism became the official religion of
Rome for some 300 years.
The early Christian church later adopted Sunday as their holy
day, and December 25 as the birthday of Jesus."
- The Philosophies and Religions of the
Roman Empire, by Dr. C. George Boeree, (http://webspace.ship.edu/cgboer/romanempire.html)