ITYM "autumnal equinox", it's on the 23rd this year.
Since the equinoxes align with the tilting of the axis, one would think that they, and the solstices, would fall in the exact center of their seasons. However, temperature lag throws this off. I.e. in California, I consider fall as September, October, and much of November, and the greatest impact it has tends to be on clothes. So, come Labor Day I shelve my summer clothes and switch to more autumn items.