Mihragan, celebrating Mithra on the 16th day of the 7th month (October 2nd).

Mihragan is one of the most ancient festivals known, dating back at least as far as the earliest Indo-Europeans. According to Taqizadeh, (1938, p. 38: "The feast of Mithra or baga was, no doubt, one of the most popular if not the greatest of all the festivals in ancient Iran, where it was celebrated with the greatest attention. This was originally a pre-Zoroastrian and old Aryan feast consecrated to the sun god, and its place in the Old-Persian calendar was surely in the month belonging to this deity. This month was called Bagayadi or Bagayadish and almost certainly corresponded to the seventh Babylonian month Tishritu, the patron of which was also Shamash, the Babylonian sun god. This month was, as has already been stated, probably the first month of the Old-Persian year, and its more or less fixed place was in the early part of the autumn. The feast was in all probability Old-Persian rather than Old- or Young-Avestan, and it was perhaps the survival of an earlier Iranian New Year festival dating from some prehistoric phase of the Aryo-Iranian [Indo-Iranian] calendar, when the year began at the autumnal equinox. It was connected with the worship of one of the oldest Aryan dieties (Baga-Mithra), of whom traces are found as far back as in the fourteenth century B.C."

In the Zoroastrian religious calendar, Mihragan is celebrated on the sixteenth day of the seventh month. According to Fasli reckoning, this occurs on October 1. Modi (1922), pg 463, states that Mihragan should properly fall on the fall equinox (which is the first day of the seventh month), but it is usually performed on the name day of Mithra (16th day). http://www.avesta.org/mihragan.htm