Mithra — like Tammuz, Adonis, Apollo and Horus — was a sun-god. So you see, the "reason for the season" is not the son of any god, but the sun up in the sky!

Most of the pagan cults are exterminated now, thanks to the love of Christ, but the celebrations continue under new management. It seems that the pagan spirit, minus the bloodshed of the executed-then-resurrected redeemer-god, lives on.