Harvest Moon is just a name. In some ways, it’s like any other full moon name. But these autumn full moons do have special characteristics, related to the time of moonrise. Nature is particularly cooperative in giving us dusk-till-dawn moonlight, for several evenings in a row, around the time of the Harvest Moon.
The Harvest Moon is either the last full moon of the summer season, or the first full moon in autumn. This year, the September full moon is the fourth of the season’s four full moons. That’s the fourth moon of summer for the Northern Hemisphere, and the fourth winter full moon for the Southern Hemisphere.
WHY IS IT CALLED THE HARVEST MOON?
For several evenings, the moonrise comes soon after sunset. This results in an abundance of bright moonlight early in the evening, which was a traditional aide to farmers and crews harvesting their summer-grown crops. Hence, it’s called the “Harvest” Moon!
It must seem like a true blessing for farmers as just when days were getting rapidly shorter and the Sun seems to go down all too soon, the Harvest Moon arrives to extend the hours that harvesting could be done.
WHEN IS THE HARVEST MOON?
This year, the brilliant Harvest Moon will appear in the evening of Monday, September 20, reaching peak illumination at 7:54 P.M. EDT.
One thing that sets the Harvest Moon apart from other full Moon names is that it’s not associated with a specific month, as the others are. Instead, the Harvest Moon relates to the timing of the autumnal equinox (September 22, 2021), with the full Moon that occurs nearest to the equinox being the one to take on the name “Harvest Moon.” This means that the Harvest Moon can occur in either September or October, depending on how the lunar cycle lines up with the Gregorian calendar.
Fred Schaaf --"Farmer's Almanac"--