Specifically, the Northern Hemisphere marks the autumnal equinox on Saturday, September 22, 2018, at exactly 8:54 P.M. CDT.

The autumnal equinox—also called the September or fall equinox—is the astronomical start of fall in the Northern Hemisphere and spring in the Southern Hemisphere.

The word equinox comes from the Latin aequus (equal) and nox (night).