Happy fall, y'all!

The autumnal equinox will officially usher in astronomical fall Monday afternoon, with 12 hours and nine minutes of daylight, along with the illuminating phenomenon known as " Chicagohenge ."

According to the Adler Planetarium, the event occurs just twice a year -- during fall and spring equinoxes -- when the sun directly lines up with Chicago's east-west facing streets.

"Keep an eye on those sunset and sunrise times," NBC 5 Storm Team Meteorologist Alicia Roman said, of when to expect to see the spectacle.

As the final hours of summer tick down, here's what to know about the fall equinox, "Chicagohenge" and more.

When does fall start?

The autumnal equinox occurs in Chicago area at 1:19 p.m. CT Monday, making the start of fall season in the Northern Hemisphere.

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