The Autumn equinox happens every year on either the 22nd, 23rd or 24th September, depending on calendar.
The term equinox comes from the Latin words aequus, meaning equal and nox, meaning night. The sun crosses the celestial equator, which is the imaginary line that marks the equator of Earth extending up to the sky. At this day the amount of nighttime and daytime are roughly equal to each other.
Aiva the Flutist wishes you all nice autumn!