The first day of fall is known as the autumnal equinox, which was September 23 in 2015. The exact date varies from year to year.
The autumnal equinox is the day when the center of the sun is directly over Earth's equator at noon and sets 12 hours after rising. The hours of daylight and night are approximately equal around the equinox, which derives from the Latin term for "equal night". There are two equinoxes per year to mark when Earth is evenly facing the sun. The rest of the year Earth is tilted on its axis and receives sunlight at an angle that determines the seasons.