Here is an explanation of why the Harvest Moon carries that name from The Inconstant Moon (enter "harvest" under Search):
"A full moon, because it is directly on the opposite side of the Earth to the Sun, always rises at about sunset and sets around sunrise. At the time of the vernal equinox, roughly 22nd September, the period of darkness between sunset and moonrise remains small for up to a week after the full moon. Conveniently, this happens at exactly the time when farmers are working round the clock to harvest their crops - a huge boon in the days before electric lights - so the full moon closest to the equinox become known as the Harvest Moon. The precise date varies between the second week in September and the first week in October."
Ilka