The full Moon that occurs nearest to the autumnal equinox is has long been called the Harvest Moon because of its relationship to farming.
Around the fall equinox, the full Moon rises around sunset for several nights in a row, which traditionally provided farmers with just enough extra light for them to finish their harvests before the killing frosts of fall set in. Normally, the Moon rises about an hour later each night, but around the time of the fall equinox, the angle of the Moon’s orbit and the tilt of the Earth line up just right and cause the Moon to rise only about 20 to 30 minutes later each night for several nights in a row.
An Astronomical Moon Name
The Harvest Moon is one of only two Moon names that are astronomical terms and aren’t tied to one specific month. Because it’s always the full Moon nearest to the equinox that’s called the Harvest Moon, either September or October’s full Moon can take on the name. (The other astronomical Moon name is the Hunter’s Moon, which is the full Moon that directly follows the Harvest Moon. It can occur in either October or November.) This year, the Harvest Moon happens on Monday, September 20—just two days prior to the autumnal equinox.
Source: Farmer’s Almanac