Mehregan (Contemporary Persian) is derived from the Mihrakān/Mihragān (Pahlavi Sassanid/Middle Persian), itself derived from Mithrakāna (Achaemenid/Old Persian), is an Iranian festival honoring the Zoroastrian yazata (angelic divinity) Mithra. Mehregan is the second most important festival, falling behind Nowruz. Due to these two festivals being heavily connected with the role of Iranian kingship, the Sasanian rulers were usually crowned on either Mehregan or Nowruz. On this day, Fereydun vanquished the evil Zahhak and confined him to Mount Damavand. This fragment of the legend is part of a greater cycle that ties Mehrgan with Nowruz; Dahak vanquished Jamshid (who the legends have as the one establishing Nowruz or New Year's Day), and Fereydun vanquishes Zahhak, so restoring the balance. On the association of Mehrgan with the polarity of spring/autumn, sowing/harvest, and the birth/rebirth cycle, Biruni the polymath wrote, "They consider Mihragān as a sign of resurrection and the end of the world, because at Mihragān that which grows reaches perfection."
