It is called Equinox, nox noctis meaning NIght. As Daily Duration varies along with Nocturnal Duration, only 2 days, Mar 21 and Sept 22 are Equinoctical.
Yadha Mesha Thulayo Vartate, Thadha Ahoratranam Samanani Bhavanti.
On those days, the Celestial Equator, Vishuvat Vritta, intersects the Kranti Vritta, the Slanted Circle, the Ecliptic.