The first day of fall is Saturday, September 23, 2023—the same day as the autumn equinox—in the Northern Hemisphere. The date varies slightly year to year, but it is usually September 22 or 23. On rare occasions, it can fall on September 21 or 24.
The first day of fall is the day of the year when the sun crosses the celestial equator moving southward. This day is known as the Autumnal Equinox. The equinox marks the end of warm summer days and the start of longer chilly nights.