Northern Hemisphere: September, October, and November.Southern Hemisphere: March, April, and May.
There are two primary ways to define the start and end dates of autumn: Why Meteorological Fall Starts Before Astronomical Fall
Autumn is generally defined as a three-month transition between summer and winter, but the specific months it encompasses depend on your location and the method of calculation used. By Hemisphere autumn is in what month
In higher latitudes, the leaves may begin to turn gold and red as early as late August. In warmer coastal regions, summer heat can linger well into October, making "autumn" feel much shorter.
In the Southern Hemisphere (countries like Australia and Brazil), the seasons are flipped. Their autumn begins at the March equinox, typically March 20 or 21, and ends in June. The Meteorological Definition Northern Hemisphere: September, October, and November
According to this system, autumn in the Northern Hemisphere occupies three full months: SeptemberOctoberNovember
Meteorologists and climatologists use a different system to keep weather records consistent. They divide the year into four three-month periods based on the annual temperature cycle. In warmer coastal regions, summer heat can linger
Here is the breakdown depending on how you define it: