What astronomical event determines the start of Nowruz in the Solar Hijri calendar?

The vernal equinox determines the start of Nowruz in the Solar Hijri calendar.

At the vernal equinox, the Sun crosses the celestial equator while moving northward, producing nearly equal day and night around the world. Nowruz begins at the precise equinox moment, and the first day of the Solar Hijri year is assigned accordingly.

This astronomical basis distinguishes the Solar Hijri calendar from purely arithmetic solar calendars and from lunar calendars. Because the equinox occurs at a specific instant, the corresponding civil date can vary by time zone and by the calendar rules used in different countries.

Nowruz is therefore not simply a festival held on a permanently fixed Gregorian date. It usually falls on March 20 or March 21, although the exact date depends on the equinox and local calendar practice.