The first day of autumn every year is September 23. This day is known as the autumnal equinox. Therefore, the last day of the summer season is September 22.
The term equinox comes from Latin words meaning "equal night." The center of the sun is visible for exactly 12 hours and below the horizon for 12 hours. The autumnal equinox is one of only two days every year that this happens, with the other being the vernal equinox, which occurs on March 21. The equinox is opposite of the solstices, which occur at the beginning of summer and winter and have an abundance and lack of light respectively.