According to the Avesta (book of unknown/hidden wisdom) “hamaß.path.maiðya” or Vernal Equinox marks the beginning of the religious year.
The Avestan term “hamaß.path.maiðya” refers specifically to the precise moment when the celestial paths (Avestan path) are at an equal/same length (hamaß) from each other and the Sun is in the center/middle position (maiðya;) in other words, the exact moment of Equinox.
The Zoroastrian religious calendar has exactly 12 months of 30 days each plus 5 gatha/song days at the end of each year.
However, a solar calendar is around 365 ¼ days, which the Seasonal Avestan calendar accommodates by adding one day every four years (a leap day called avardaad,) or an extra month every 120 years as prescribed in Denkart (III.419.)
The qadimi and shänšaaii are respectively folk Zoroastrian Iranian and Parsi calendars that did NOT add an extra day in leap years and/or a 13th month each 120 years; as required by holy Denkart. Hence these folk calendars moved into mid-summer.
It appears that the last time that the Parsi Nauv-rooz coincided with the Vernal equinox was about 600 years ago according to the Parsi Shänšaaii reckoning. The last time that Iranian Zoroastrian qadimi coincided with vernal equinox was about 500 years ago.
The difference of a month between Iranian qadimi (ancient) and Parsi royal calendar could be explained in term of implementation of an extra 13 month for another century or so in Iran.
An almost identical calendar to that of folk Zoroastrian qadimi or Parsi Calendar is still in use in the lush Caspian Mountains of Northern Iran.
This is due to the fact the Caspian province of Mazandaran was ruled by a Zoroastrian dynasty until 1597. Their downfall occurred during the rule of Shah Abbas the Safavid (1587-1628 CE.)
The Mazdandarani Nauvrooz (East Caspian) is celebrated around the 24 or 25 July.
The Gilani Nauvrooz (West Caspian) known also as Nauvrooz Bal or Nowrouz Bal is celebrated on 6 or 7 August.
The term Bal refers to the “brilliancy of fire” and is translated as the Nauvrooz of “brilliant fire.”
Also, along the Persian Gulf coast, there is a Nauvrooz -e daryaaii (Seafaring Nauvrooz) celebrated on 31 July or 1 August.
All these celebration are reminiscent of the last Zoroastrian enclaves/strongholds who were forcefully converted during the cruel Safavid rule.
In conclusion, I shall add that in most sections in Persian astronomical works, which describe various calendars, one comes across a Nauv-rooz-e bozôrg (Great Nowruz) on the 6th day after Nauvrooz. The “Great Nauv-rooz” is also called Nauv-rooz -e ḵordáḏi (Nowruz of the day of Ḵordád.
Ḵordád (Avestan haûrvatát, “every healing power, remedy”) is the name given to the 6th day of every ancient Iranian month. Nauv-rooz-e bozôrg or ḵordáḏi marks the birthday of ancient Aryan seer/prophet Zarathûštrá, a day of healing, restoration of all life powers, growth and new rebirth.