At 08:49 MDT (just under an hour ago, as this post goes up), the Sun crossed the celestial equator heading north. It's autumn! Today there will be more or less equal periods of daylight and darkness, and from then on, the hours of night will exceed the hours of daylight.
The first days of the other three seasons carry spiritual significance. The Feast of the Annunciation (March 25) coincides with the Vernal Equinox, marking the end of the winter of Satan's dominion over man. The Nativity of John the Baptist (June 24) coincides with the Summer Solstice, when the days begin to grow shorter -- the decrease of John the Baptist, while Christ increases. Thus, Christmas (December 25) coincides with the Winter Solstice, when the days begin to lengthen.
What about the Autumnal Equinox? There are no cataclysmic events commemorated at this time (though the Feast of Our Lady of Mercy does fall on the 24th). But autumn is the season of harvest, so perhaps -- and this is only my speculation -- the Autumnal Equinox stands for the end of time, when the harvest of souls is consummated. Perhaps this is an opportune moment for each one of us to consider whether we are ready for that unknown yet inevitable moment when we are to be harvested.