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.

The average date for a first measurable snow in Philly is Nov. 21. Normally, just over 2 inches of snow has fallen upon Philly officially before the winter solstice. The earliest snow, 2.2 inches of it, occurred on Oct. 10, 1979.

The first day of fall, also known as the autumnal equinox, is Sunday, Sept. 22, at 8:44 a.m. ET. While the equinox is known as the astronomical start of the season, there is another method used to measure the change in seasons. The meteorological beginnings of seasons are based on the annual temperature cycle and the 12-month calendar.