The Zoroastrian festival associated with the autumn equinox is Mehregan.
Mehregan, also called Jashn-e Mehr or Mihragan, is an Iranian festival honoring Mithra, the Zoroastrian yazata associated with covenants, friendship, affection, and light. In the traditional seasonal cycle it stands opposite Nowruz, the spring-equinox festival, and is linked with autumn, harvest, ripening, and the restoration of order.
The festival has deep royal and legendary associations. In Iranian tradition, it commemorates the hero-king Fereydun’s victory over the tyrant Zahhak. During the Achaemenid period, Armenian subjects reportedly brought thousands of horses to the Persian king during the celebration. Under the Sasanians, Mehregan ranked second only to Nowruz in importance.
Dates can vary because Zoroastrian communities use different calendar systems, so modern observances may fall around late September or October rather than on the astronomical equinox itself. Sadeh is the winter fire festival, Tirgan is associated with summer, and Nowruz belongs to spring.