The Harvest Moon is the full Moon nearest the start of fall or the autumnal equinox. This usually means it’s the September full Moon though it can also fall in early October, coming anywhere from two weeks before to two weeks after the equinox.
There are just a fraction over 12 complete Moon cycles every year, on average (there being about 29.53 days in a synodic month). The Harvest Moon isn’t like the other Moons. Usually, throughout the year, the Moon rises an average of about 50 minutes later each day. But near the autumnal equinox, the difference is only 30 minutes.
Also,the Full Harvest Moon rises at sunset and then will rise near sunset for several nights in a row because the difference is at a yearly minimum. It may almost seem as if there are full Moons multiple nights in a row!
The abundance of bright moonlight early in the evening was a traditional aide to harvest crews, hence the “Harvest” Moon. Now you know!