Astronomical fall vs. meteorological fall
There are various ways to explain the start and end dates of fall. The two most commonly used are based on the astronomical calendar and the meteorological calendar.
In the Northern Hemisphere summer ends and fall starts during September equinox, which occurs every year on September 21 to 23. Fall ends and winter starts at the moment of the December solstice, which occurs every year between December 20 and December 23. The starting and ending dates of fall are based on the changing position of Earth in relation to the sun, and then results of solar events of equinoxes and solstices.
Meteorological fall in most of the Northern Hemisphere is usually defined as the three months of September, October and November, starting with September 1st and ending on November 30th.