Fall starts Tuesday, according to meteorologists. Meteorological fall (aka autumn) is defined as the months of September, October and November in the Northern Hemisphere (it's spring in the Southern Hemisphere). Most people consider the first day of autumn to be the autumnal equinox, which this year is September 23, according to WeatherBug meteorologist Mark Hoekzema. This is because, astronomically, the sun will be directly overhead of the equator as the Earth`s tilt begins to point the Northern Hemisphere away from the sun.