Although it is not clear whether proto-Indo-Iranians celebrated a feast as the first day of the calendar, there are indications that Iranians may have observed the beginning of both autumn and spring, respectively related to the harvest and the sowing of seeds, for the celebration of the New Year. Boyce and Grenet explain the traditions for seasonal festivals and comment: "It is possible that the splendor of the Babylonian festivities at this season led the Iranians to develop their own spring festival into an established New Year feast, with the name Navasarda "New Year" (a name which, though first attested through Middle Persian derivatives, is attributed to the Achaemenian period)." Since the communal observations of the ancient Iranians appear in general to have been a seasonal ones, and related to agriculture, "it is probable that they traditionally held festivals in both autumn and spring, to mark the major turning points of the natural year."
Nowruz is partly rooted in the tradition of Iranian religions, such as Mitraism and Zoroastrianism. In Mitraism, festivals had a deep linkage with the sun light. The Iranian festivals such as Mehrgan (autumnal equinox), Tirgan, and the eve of Chelle ye Zemestan (winter solstice) also had an origin in the Sun god (Surya). Among other ideas, Zoroastrianism is the first monotheistic religion that emphasizes broad concepts; such as the corresponding work of good and evil in the world, and the connection of humans to nature. Zoroastrian practices were dominant for much of the history of ancient Iran. In Zoroastrianism, the seven most important Zoroastrian festivals are the six Gahambar festivals and Nowruz, which occurs at the spring equinox. According to Mary Boyce, "It seems a reasonable surmise that Nowruz, the holiest of them all, with deep doctrinal significance, was founded by Zoroaster himself"; although there is no clear date of origin. Between sunset on the day of the sixth Gahambar and sunrise of Nowruz, Hamaspathmaedaya (later known, in its extended form, as Frawardinegan; and today known as Farvardigan) was celebrated. This and the Gahambars are the only festivals named in the surviving text of the Avesta.
The 10th century scholar Biruni, in his work Kitab al-Tafhim li Awa'il Sina'at al-Tanjim, provides a description of the calendar of various nations. Besides the Iranian calendar, various festivals of Greeks, Jews, Arabs, Sabians, and other nations are mentioned in the book. In the section on the Iran calendar, he mentions Nowruz, Sadeh, Tirgan, Mehrgan, the six Gahambars, Farvardigan, Bahmanja, Esfand Armaz and several other festivals. According to him, "It is the belief of the Iranians that Nowruz marks the first day when the universe started its motion." The Persian historian Gardizi, in his work titled Zayn al-Akhbār, under the section of the Zoroastrians festivals, mentions Nowruz (among other festivals) and specifically points out that Zoroaster highly emphasized the celebration of Nowruz and Mehrgan.