The most startling similarity is that Mithra and Jesus share the exact same birth date. In this case it is fairly clear that Mithra came first. There are many records that conclusively prove December 25th was first the Dies Natalis Solis Invicti, or Birth of the Unconquered Sun. It is around this date every year that the Winter Solstice occurs in the Western hemisphere, marking the shift from shorter daylight hours to longer ones. As such, this feast celebrates how the Unconquered Sun (Son?) ceases to wane in strength, returning to his full power and glory.
Saturnalia and Dies Natalis Solis Invicti, were very popular feasts of the Roman world as Christianity came onto the international stage. When Constantine adopted Christianity as the faith of the empire, it is not surprising that the most beloved festivals of the Roman people would find some place in the world of Christianity.
Does this somehow sully the idea of Christmas? In my humble opinion, it does not. Christmas is made all the more important and relevant to the world because it has roots in primordial soil. That Christmas has taken the beloved traditions of the ancient world and married them with the message of Christianity provides modern civilization with a precious link to our earliest social traditions.
