The Harvest Moon normally occurs in September, but because of a quirk this year, it's happening in October. The Harvest Moon is the moon nearest to the Autumnal Equinox. This year the Autumnal Equinox was on September 22nd.
Because of this, the Harvest Moon will occur on Oct. 4, 2:10 a.m. EDT. September's full Moon took place on the Sept. 4th, a little more than 18 days before the Autumnal Equinox. October's full Moon, on the other hand, falls a little more than 11 days after the equinox. This is therefore closer to the equinox by by a week.
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. The moon rises approximately 30 minutes later, from one night to the next, rather than the 50 minutes that is normal.
The next time the Harvest Moon appears in October won't be until 2017. So celebrate this October Harvest Moon. It will be 8 years until the next.