Fall begins in the Northern Hemisphere on Monday, Sept. 22, when Earth experiences equal day and night, known as the autumnal equinox. While fall officially begins with the autumnal equinox, if you ask a meteorologist, fall starts three weeks earlier on Sept. 1. The three months in the Northern Hemisphere between the coldest and the warmest months, summer and winter, are what define meteorological fall from Sept. 1 to Nov. 30. "The equinox is just when neither the Northern nor the Southern Hemisphere is pointed towards the Sun," Richey-Yowell said. According to NASA, this year the autumnal equinox happens on Sept. 22 at 2:19 p.m.