By definition, the harvest moon is the first full moon that occurs after the autumnal equinox. This year, the autumnal equinox falls on Saturday, Sept. 22, 2012. This would make the harvest moon this year the full moon that occurs on Sept. 29 and 30.
Besides being the first full moon after the equinox, the harvest moon gets its name for another reason. Not all full moons are created equal. Much like the sun, the moon will rise and fall each day. However, the moon does this with a much larger variability than the sun does. Whereas the sun's rise and set is at most a couple minutes different from one day to the next, the moon rises on average 50 minutes later each day than the previous day.
Unlike other full moons, it just so happens that the harvest moon's rise corresponds with sunset more closely then at any other time during the year. This means that just as night falls, the full moon begins to rise above the horizon in all it's glory. It's called the harvest moon then because it provides ample light for farmers to bring in the harvest well after the sun sets.