I just found out that

The Harvest Moon is a help to farmers anxious to complete the garnering of their crops. It is the full moon which occurs nearest the date of the autumnal equinox. It is not brighter than other full moons, as some seem to believe. Its virtue lies in the fact that, for several nights, less time elapses between its successive risings than for any other full moon of the year. This gives farmers an extra-long sequence of evenings with bright moonlight.