Harvest Moon is the name given to the full Moon closest in date to the first day of autumn, known as the autumn equinox in the northern hemisphere. This is when day and night are the same length.
The full Moon on 20 September will be the closest full Moon to the Northern Hemisphere’s autumn equinox – this will be the Harvest Moon for 2021. The Harvest Moon is the most famous of all the Full Moon names. Some sources claim the name originates from ancient Native American tradition. Others point out that Harvest month was recorded as early as in the 700s in both Anglo-Saxon and Old High German languages.
In ancient times, it was common to track the changing seasons by following the lunar month rather than the solar year, which the 12 months in our modern calendar are based on.