It is the summer's great last heat,
It is the fall's first chill: They meet.
–Sarah Morgan Bryan Piatt
What is the Autumnal Equinox?
Fall begins on September 23, early in the morning at 4:21 A.M. The autumnal equinox is when the Sun appears to cross the celestial equator from north to south. (The celestial equator is the circle in the celestial sphere halfway between the celestial poles. It can be thought of as the plane of Earth's equator projected out onto the sphere.)
Another definition of fall is nights of below-freezing temperatures combined with days of temperatures below 70 degrees Fahrenheit.
The word equinox means "equal night"; night and day are about the same length of time. This occurs two times each year: Vernal in late March and Autumnal in late September.
In addition to the (approximately) equal hours of daylight and darkness, the equinoxes are times when the Sun's apparent motion undergoes the most rapid change. Around the time of the equinoxes, variations in the position on the horizon where the Sun rises and sets can be noticed from one day to the next by alert observers.
From here on out, the temperatures begin to drop and the days start to get shorter than the nights (i.e., hours of daylight decline).