Fall Equinox

The Harvest moon comes before the fall equinox. But that is...? The Autumnal Equinox is the point in the second part of the year where the center of the sun's rays are at the equator. At this time of year the day and night are roughly equal in length which is the meaning of Equinox, equal night, in Latin. However, only people in the northern hemisphere call this time of year the fall equinox, in the southern hemisphere it is called the vernal, spring, equinox because the seasons are reversed opposite of the equator.

Source: http://www.timeanddate.com/calendar/september-equinox.html