The first day of the Iranian calendar falls on the March equinox, the first day of spring, around March 21. In the 11th century CE the Iranian calendar was reformed in order 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 Tusi was the following: "the first day of the official New Year [Nowruz] was always the day on which the sun entered Aries before noon." Nowruz is the first day of Farvardin, the first month of the Iranian solar calendar.

The word Nowruz is a combination of Persian words نو now—meaning "new"—and روز ruz—meaning "day". Pronunciation varies among Persian dialects, with Eastern dialects using the pronunciation [nawˈɾoːz] (as in Dari and Classical Persian, whereas in Tajik, it is written as "Наврӯз" Navröz), western dialects [nowˈɾuːz], and Tehranis [noːˈɾuːz]. A variety of spelling variations for the word nowruz exist in English-language usage, including novruznowruznauruz and newroz.