The September equinox on 23rd September marks the first day of autumn for anybody living in the northern hemisphere and the first day of spring for anybody living in the southern hemisphere. On the day of the equinox, everywhere on Earth has almost exactly 12 hours of day and night, as the Sun's annual journey through the constellations of the zodiac carries it across the celestial equator.
The Moon will be located on the opposite side of the Earth as the Sun and its face will be will be fully illuminated on 25th September. In the USA, this full moon was known by early Native American tribes as the Full Corn Moon because the corn is harvested around this time of year. This moon is also known as the Harvest Moon. The Harvest Moon is the full moon that occurs closest to the September equinox each year.
Sources: In The Sky by Dominic Ford, NASA, The Watchers
Image credit: https://watchers.news/2018/09/01/night-sky-guide-for-september-2018/