Wiki says- The Harvest Moon comes soon before or soon after the autumnal equinox. It is simply the full moon closest to that equinox. About once every four years it occurs in October (in the northern hemisphere), depending on the cycles of the moon. Currently, the latest the Harvest Moon can occur is on October 7.
In 2010, in the United States, the Harvest Moon happens in the early morning hours of Sept 23, only 5 1/2 hours after the autumnal equinox.
Often, the Harvest Moon seems to be bigger or brighter or more colorful than other full moons. The warm color of the moon shortly after it rises is caused by light from the moon passing 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 one's eyes. Hence all celestial bodies look reddish when they are low in the sky.
The apparent larger size is because the brain perceives a low-hanging moon to be larger than one that's high in the sky. This is known as a moon illusion and it can be seen with any full moon. It can also be seen with constellations; in other words, a constellation viewed low in the sky will appear bigger than when it is high in 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 half moons, the time difference between moonrise on successive evenings is shorter than usual which means that 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 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—the plane of Earth's orbit around the sun—makes a narrow angle with respect to the horizon in the evening in autumn.