Mithraism is connected to Libra or Mehr مهر. To be born in Libra or Mehr مهر you are conceived in Capricorn or Day دی. Winter Solstice or Yalda یلدا is the oldest festival. This is the original reason. Not what Ancient Origins and April Holloway are saying. Study the ancient cosmology of Iran. The reason the 25th is used, is because it is exactly a week before January 1st on the Gregorian calendar. The correct calendar is the Ahuramazdan astrological calendar. Originally January was in the Spring.
https://en.m.wikipedia.org/wiki/Calendar_(New_Style)_Act_1750
Originally from:
Ahuramazdan Astrology نجوم اهورامزدان 