Nowruz usually occurs in March on the Gregorian calendar.
The holiday begins at the moment of the vernal equinox, when day and night are approximately equal in length. Because the equinox can occur at different times, Nowruz generally falls on March 20 or March 21, though local calendars and time zones can affect the civil date.
Unlike a purely fixed-date festival, Nowruz is tied to an astronomical event. This is why its date is stable within the spring season but can shift by one day in the Gregorian calendar.
The Solar Hijri calendar begins with the month of Farvardin at Nowruz. Its year is solar, not lunar, so the new year remains aligned with the northern-hemisphere spring season. This distinguishes Nowruz from holidays based on a lunar calendar.