The start of Astronomical Fall begins at 1:19 pm CDT for us in the Central Time Zone on Monday, September 22nd. The beginning of Astronomical Fall marks the moment in time when the Sun is exactly above the Equator and equidistant from both poles. At this time the Earth's rotational ais is directly perpendicular to the Sun-Earth line, tilting neither toward nor away from the Sun. As a result, the day and night are of equivalent length.
Note - Meteorological Fall season begins on September 1st. The beginning of each season has more to do with temperature cycles with Summer being the warmest 3 months, Winter being the coldest 3 months and spring taking the 3 months between winter and summer and fall taking the three months between summer and winter.