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The first day of fall is on Saturday, Sept. 22, it will officially be autumn for the Northern Hemisphere, according to the U.S. Naval Observatory.

Sept. 22 is the first day of fall because it is when the autumnal equinox — the moment when “a planet’s subsolar point passes through its equator,” according to National Geographic — takes place. In layman’s terms, the autumnal equinox is when the Earth’s equator passes through the sun.