Mithraism and Christianity formed about the same time. I wouldn't expect you to believe Christianity borrowed anything from Mithraism (Sunday, December 25th, whatnot) because you are renowned for dismissing evidence that disagrees with your particular world view.
Mithraism Predates Christianity http://en.wikipedia.org/wiki/Mithras
Mithras was the central savior god of Mithraism, a syncretic Hellenistic mystery religion of male initiates that developed in the Eastern Mediterranean in the 2nd and 1st centuries BC and was practiced in the Roman Empire from the 1st century BC to the 5th century AD. Parthian coins and documents bear a double date with a 64 year interval that represents Mithra's ascension to heaven, traditionally given as the equivalent of 208 BC, 64 years after his birth.