Today was the first day of Autumn. Today we have equal periods of light and darkness -- hence the term "equinox" -- and after today, the periods of darkness will exceed in length the periods of light.
It is unfortunate that, based on no discernable mandate from the Second Vatican Council, the post-conciliar Church tried to throw out the Ember Days, penitential days that mark the changing seasons. The purpose of the Ember Days is to thank God for His bounty in nature, and to remind us to use His gifts in moderation and assist those in need. As I have pointed out in this space, the changes of season are freighted with spiritual significance. The vernal equinox coincides with the Feast of the Annunciation, the beginning of the end of the winter of Satan's reign. It also coincides with Easter, which marks the decisive defeat of Hell, and takes place on the first Sunday on or after the first full moon on or after the equinox. The summer solstice, when the days begin to shorten, coincides with the Nativity of John the Baptist, who said that he must decrease while the Savior increased. The winter solstice coincides with Christmas, when the Light of the World enters the world and the days begin to lengthen.
There is no precise correspondence between the autumnal equinox and any major feast; but since the autumnal equinox does coincide with the time of harvest, my own personal speculations lead me to connect it with the harvest of souls that will take place at the End of Time. Our business in life is to strive to come out on the right side of that harvest: to be in with the wheat that is gathered into the barn, and not with the tares that are bundled up and go to be burned.