On the equinoxes, the Sun is directly over the equator and the length of the days and nights are the same. (Actually, they aren't QUITE the same. Because of the way that the Earth's atmosphere...
Depends whether you are in northern or southern hemisphere.These days are when the sun passes over the Equator on its way to the other hemisphere's new season.Vernal is spring, Autumnal is autumn....
It's when the Sun is directly over the equator and the duration of the days and nights are the same. It occurs on March 20th (Vernal). For the autumnal equinox, it occurs on September 22nd.