The harvest moon is the moon at and about the period of fullness that is nearest to the autumnal equinox. - Dictionary.com

The Harvest moon is a legend in Norse mythology, it is said to be the most powerful of the Moons granting Loki's blessing for a strong harvest and plenty. The Harvest moon is often mistaken for the modern day Hunter's moon.

In the legend of the Harvest moon, it is said that all full moons have their own special characteristics based primarily on the whereabouts of the ecliptic in the sky at the time of year that these moons are visible. The full moons of September, October and November as seen from the northern hemisphere - which correspond to the full moons of March, April and May as seen from the southern hemisphere - are well known in the folklore of the sky. All full moons rise around the time of sunset. However, although in general the moon rises about 50 minutes later each day, as it moves in orbit around Earth, the Harvest Moon and Hunter's Moon are special, because around the time of these full moons, the time difference between moonrise on successive evenings is shorter than usual. In other words, the moon rises approximately 30 minutes later, from one night to the next, as seen from about 40 degrees N. or S. latitude, for several evenings around the full Hunter's or Harvest Moons. Thus there is no long period of darkness between sunset and moonrise around the time following these full moons. In times past this feature of these autumn moons was said to help farmers working to bring in their crops (or, in the case of the Hunter's Moon, hunters tracking their prey). They could continue being productive by moonlight even after the sun had set. Hence the name Harvest (or Hunter's) Moon.

The reason for the shorter-than-usual rising time between successive moonrises around the time of the Harvest and Hunter's Moon is that the ecliptic - or plane of Earth's orbit around the sun - makes a narrow angle with respect to the horizon in the evening in autumn.

The Harvest Moon is said to come before or after the autumnal equinox. It is simply the full moon closest to that equinox. About once every four years it occurs in October, depending on the cycles of the moon. Currently, the latest the Harvest Moon can occur is on October 8. Between 1900 and 2010 the Harvest Moon falls on October 7 in 1930, 1949, 1987, 2006, and on October 8 in 1911.

Norse mythology celebrates with gatherings, festivals, and rituals that are intricately attuned to the Harvest Moon or Hunter's Moon.

Often, the Harvest Moon seems to be bigger or brighter or more colorful than other moons. These effects have to do with the seasonal tilt of the earth. The warm color of the moon shortly after it rises is an optical illusion, based on the fact that when the moon is low in the sky, you are looking at it through a greater amount of atmospheric particles than when the moon is overhead. The atmosphere scatters the bluish component of moonlight (which is really reflected white light from the sun), but allows the reddish component of the light to travel a straighter path to your eyes. Hence all moons (and stars and planets) look reddish when they are low in the sky.