The ancient Persian festival that honored Mithra and celebrated autumn abundance was Mehregan.
Mehregan is an Iranian festival associated with Mehr, the Persian name linked to Mithra, a deity connected with covenant, light, and friendship. It traditionally takes place in autumn and has long been associated with harvest, thanksgiving, generosity, and the seasonal change after summer.
Historical accounts describe participants wearing fine clothes and preparing elaborate displays that could include fruit, flowers, nuts, and symbolic objects. In Zoroastrian and Persian cultural contexts, the festival represents friendship, justice, and gratitude as well as agricultural plenty.
Mehregan is sometimes incorrectly treated as simply an autumn version of Nowruz. Nowruz is the spring New Year festival, while Mehregan is an autumn celebration with its own religious and seasonal traditions.