Due to its antiquity, there exist various foundation myths for Nowruz in Iranian mythology. In the Zoroastrian tradition, the seven most important Zoroastrian festivals are the six Gahambars and Nowruz which occurs at the spring equinox. According to the late Professor Mary Boyce: It seems a reasonable surmise that Nowruz, the holiest of them all, with deep doctrinal significance, was founded by Zoroaster himself. Between sunset of the day of the 6th Gahanbar and sunrise of Nowruz was celebrated Hamaspathmaedaya (later known, in its extended form, as Frawardinegan). This and the Gahanbar are the only festivals named in the surviving text of the Avesta.
The Shahnameh, dates Nowruz as far back to the reign of Jamshid, who in Zoroastrian texts saved mankind from a killer winter that was destined to kill every living creature. The mythical Persian King Jamshid (Yima or Yama of the Indo-Iranian lore) perhaps symbolizes the transition of the Indo-Iranians from animal hunting to animal husbandry and a more settled life in human history. In the Shahnameh and Iranian mythology, he is credited with the foundation of Nowruz. In the Shahnama, Jamshid constructed a throne studded with gems. He had demons raise him above the earth into the heavens; there he sat on his throne like the sun shining in the sky. The world's creatures gathered in wonder about him and scattered jewels aound him, and called this day the New Day or No/Now-Ruz. This was the first day of the month of Farvardin (the first month of the Persian calendar).
The Persian scholar Abu Rayhan Biruni of the 10th century A.D., in his Persian work "Kitab al-Tafhim li Awa'il Sina'at al-Tanjim" provides a description of the calendar of various nations. Besides the Persian calendar, various festivals of Arabs, Jews, Sabians, Greeks and other nations are mentioned in this book. In the section on the Persian calendar(تقویم پارسیان), he mentions Nowruz, Sadeh, Tiregan, Mehregan, the six Gahanbar, Parvardegaan, Bahmanja, Isfandarmazh and several other festivals. According to him: It is the belief of the Persians that Nowruz marks the first day when the universe started its motion.
Bas-relief in Persepolis - a symbol Zoroastrian Nowruz - in day of a spring equinox power of eternally fighting bull (personifying the Earth), and a lion (personifying the Sun), are equal