The autumnal equinox is one of the two days during the year when day and night are approximately equal, and days begin to get shorter. For the northern hemisphere, this is the third week of September, the first day of autumn according to our calendars. For the southern hemisphere, the autumnal equinox occurs in March, as the seasons run opposite the calendars of the northern hemisphere. You can find more information here: http://scienceworld.wolfram.com/astronom...

An equinox is when the tilt of the Earth's axis has the sun directly over the equator. This happens twice per year. The Vernal (spring) Equinox happens around the 20th or 21st of March and the Autumnal (fall) Equinox happens around the 22nd or 23rd of September. Because the sun is directly over the equator there is a roughly equal amount of daylight and darkness over the entire planet during each equinox.