Nowruz marks the first day of spring and the beginning of the year in Iranian calendar. It is celebrated on the day of the astronomical vernal equinox, which usually occurs on March 21 or the previous/following day depending on where it is observed. As well as being a Zoroastrian holiday and having significance amongst the Zoroastrian ancestors of modern Iranians, the same time is celebrated in the Indian sub-continent as the new year. The moment the Sun crosses the celestial equator and equalizes night and day is calculated exactly every year and Iranian families gather together to observe the rituals.
Nowruz and the spring equinox
The first day on the Iranian calendar falls on the March equinox, the first day of spring. At the time of the equinox, the sun is observed to be directly over the equator, and the north and south poles of the Earth lie along the solar terminator; sunlight is evenly divided between the north and south hemispheres.
In ca. 11 century CE major reforms of Iranian calenders took place and whose principal purpose was to fix the beginning of the calendar year, i.e. Nowruz, at the vernal equinox. Accordingly, the definition of Nowruz given by the Iranian scientist Ṭūsī was the following: “the first day of the official new year [Nowruz] was always the day on which the sun entered Aries before noon”