The Autumn Equinox marks the start of autumn in the Northern Hemisphere, ushering in shorter days and longer nights. The equinox is the astronomical start of autumn, marked by the Earth’s tilt and the Sun’s position in the sky. The date varies from year to year but typically falls on one of four days towards the end of September. By meteorological standards, however, the first day of autumn kicks off this weekend on Sunday, September 1.