The official first day of fall, otherwise known as the autumnal equinox, is this weekend in the Northern Hemisphere. According to Encyclopedia Britannica, it is one of only two days, the other being the vernal equinox, when the sun is directly above the Equator, essentially dividing night and day into equal portions.

Sept. 22, marks the first day of the autumnal equinox in the Northern Hemisphere, according to the Farmer’s Almanac. It begins approximately at 8:43 a.m. ET. Meanwhile, meteorological fall begins on Sept. 1 and makes it easier to keep seasonal weather records, according to FOX Weather.

According to Britannica, the fall equinox is one of two moments in the year when the sun is exactly above the equator. The celestial event happens annually around Sept. 22 or Sept. 23 in the Northern Hemisphere and on March 19 or March 20 in the Southern Hemisphere.