When is the Harvest Moon?
The Harvest Moon is the full Moon nearest the start of fall or the autumnal equinox. This usually means it coincides with the September full Moon, though it can also fall nearer to the October full Moon, occurring anywhere from two weeks before to two weeks after the equinox.
In 2018, the Harvest Moon will rise on Monday, September 24 at 10:52 P.M. (EDT).
(From "The Old Farmer's Almanac")