The Harvest Moon gets its name because it is the full Moon closest to the equinox on September 22 this year.
The equinox is an astronomical event in which the Sun crosses the celestial equator - the projection of the Earth’s equator into space.
On the date of the equinox, both day and night are around the same length around the globe. Afterwards, the night is longer than the day.
The autumnal equinox is associated with the harvest. Harvest festivals are traditionally held on the Harvest Moon. And don't forget there is also a lunar eclipse tonight as well. Should be astounding.