A Harvest Moon happens once every year, because it’s simply the full moon that falls closest to the autumnal equinox.

What a Harvest Moon Is

  • The Harvest Moon is the full moon nearest the autumnal (fall) equinox, which occurs around September 22–23 in the Northern Hemisphere.
  • That means there is one Harvest Moon per year, though its calendar date can shift between early September and early October.

When It Usually Occurs

  • In most years, the Harvest Moon falls in September, replacing the usual September full-moon name (like Corn Moon).
  • Roughly every three years, the Harvest Moon lands in October instead, taking the place of the Hunter’s Moon for that year.

Why It Feels “Special”

  • Around the equinox, the moonrise times change very slowly from night to night, so the nearly full moon rises close to sunset for several evenings in a row, giving long, bright evenings.
  • This repeating evening brightness is what historically helped farmers bring in crops and gave the Harvest Moon its name and cultural significance.

TL;DR: A Harvest Moon occurs once each year, as the full moon closest to the autumnal equinox, usually in September but sometimes in October.

Information gathered from public forums or data available on the internet and portrayed here.