I found it interesting that there is still so much confusion about what Harvest Moon means. Many sites including NASA seem to think it is just the full Moon in September but it isn’t quite that simple. In a terribly oversimplified explanation what you are seeing is the solar calendar (the months) and the lunar calendar not lining up very well. The following text is from the Farmer’s Almanac:
The Harvest Moon is the one that occurs the closest to the Autumnal Equinox so this year it occurs in September, although occasionally this title can be bestowed upon the October full Moon. From 1970 to 2020 this happens twelve times and, in fact, will happen in 2017. The 2016 version of the Harvest Moon comes six days prior to the Autumnal Equinox, although it can occur as early as September 8 (as in 2014) or as late as October 7 (as in 1987).