Illinois state climatologist Jim Angel explains, it depends:
No doubt today (September 22) will be announced as the “first day of fall” because of the fall or autumnal equinox. However, that concept refers to the date when we get equal amounts of daylight and dark.
Climatologists and meteorologists prefer to use calendar months to define the four seasons in the US. For example, fall would start September 1 and end on November 30. Not only is this more convenient, because you can use monthly data, but it lines up better with the typical or average temperature pattern for Illinois. Unfortunately, the meteorologists would describe this three-month period as “meteorological fall”. However, I would argue it is “climatological fall” since we are looking at long-term average to determine the season.
In summary, while the four equinox and solstice events are interesting, they are not really the best way to define the start of seasons in Illinois. Starting dates of March 1 for spring, June 1 for summer, September 1 for fall, and December 1 for winter are better aligned with the climatological data.
But if you prefer to use the September equinox as your official beginning of fall, that event will take place at 21:29 CDT (02:29 UTC) tonight.
And for you pagans out there, may you have a balanced and warm equinox.