September 1st, the first day of fall.
The definition of autumn from Merriam-Webster:
the season between summer and winter comprising in the northern hemisphere usually the months of September, October, and November or as reckoned astronomically extending from the September equinox to the December solstice
I grew up with the astronomical definition for seasons–with equinoxes and solstices–but a calendar or monthly definition seems to match the change of seasons better. After a June-July-August that saw many days in the upper 90s and 100s here in Bucharest, Romania, I’m ready for a change of seasons.
Grace and Peace