Equinox.
Either of the two points at which the celestial equator intersects the ecliptic, and also the times when the Sun passes through either of these points. The Sun passes from south to north at the northern vernal (spring) equinox and from north to south at the northern autumnal equinox. The approximate dates are 21 March and 23 September.
The position of the northern vernal equinox is also traditionally known as ิthe first point of Aries'. However, the effects of precession have gradually moved the point so that it now actually lies in the adjacent constellation of Pisces.