
The name equinox means "equal night" in Latin. It's theoretically the day of the year when all points on the earth's surface experience the same lengths of daylight and darkness - 12 hours of each. The autumn equinox in the northern hemisphere (it's the spring equinox for the southern hemisphere) always falls on 22, 23 or 24 September.
http://www.bbc.co.uk/news/magazine-34334712