The first day of Fall is September 23, 2023. The word Equinox comes from the Latin word meaning equal nights. On September 23rd, the day and night are each about 12 hours long in the Northern Hemisphere, occurring a few days after the autumnal equinox. The Spring and Fall equinoxes, are the only days the Sun crosses the celestial equator going southward; it rises exactly due east and sets exactly due west. From here on out, the temperatures will start to drop and the days start to get shorter than the nights. And, we can start looking for the vibrant fall colors.
The four seasons are determined by changing sunlight (not heat!) and how our planet orbits the Sun and the tilt of its axis.