The Harvest Moon is just a specific full moon (to mark harvest time). It's in September, I think late September. And yes, a Blue Moon is the 2nd full moon within a calendar month. Edit: Was finding out specifics on the time of the Harvest Moon (late Sept-early Oct, the full moon closest to the Autumnal equinox) and found this on the Blue Moon: The third full moon in a season with four full moons is called a blue moon, as described in the Maine Farmer's Almanac. Until recently it was commonly misunderstood that the second full moon in a month was the blue moon. However, it was recently discovered by Sky & Telescope Magazine and reported on NPR that the interpretation of a blue moon as the second full moon of the month was erroneously reported in an issue of Sky & Telescope dating back to 1946 and then perpetuated by other media.
