Late September, October, November, and most of December.
According to this definition:
The season is unique because it is the only one with two widely accepted names in the English language. what months are the fall
So when someone asks "what months are the fall" using the astronomical system, the answer is: late September, all of October and November, and most of December (up to the solstice).
Historically, autumn is the primary season for harvesting crops, ranging from pumpkins and apples to corn and squash. This is heavily reflected in cultural celebrations like Thanksgiving and harvest festivals. Late September, October, November, and most of December
In the Northern Hemisphere, astronomical fall begins on the and ends on the Winter Solstice . September: Fall begins between September 21st and 23rd. December: Fall ends between December 20th and 23rd. The Science of the Equinox
Starts at the March equinox (March 20–21) and ends at the June solstice (June 20–21). Historically, autumn is the primary season for harvesting
Let's clear up a few frequent points of confusion:
Scientists, weather forecasters, and astronomers actually define the seasons using two different methods. This creates slightly different start and end dates for the autumn season. Meteorological Fall (Based on the Calendar)