The Harvest Moon for 2013 falls on 19 September. Each full moon of the twelve months has a folkloric name of its own and that for the month of September is called the “Harvest Moon”. According to NASA, the Harvest Moon gets its name from farmers who relied on the moon and its celestial schedule to harvest their crops. Since most crops ripen in late summer and early autumn, farmers would have to harvest during this time of the year (in the northern hemisphere, this is September, but not in the southern hemisphere).
“In the days before electric lights, farmers depended on bright moonlight to extend the workday beyond sunset,” writes NASA’s Dr. Tony Phillips. “It was the only way they could gather their ripening crops in time for market. The full moon closest to the autumnal equinox became the Harvest Moon, and it was always a welcome sight.”
Info from NASA.