September is the time of the Autumn Equinox - that time of year when the day and night are of (almost) equal duration, and marks the start of Autumn (fall) in the Northern Hemisphere.
An equinox is commonly regarded as the moment when the plane (extended indefinitely in all directions) of Earth's equator passes through the center of the Sun, which occurs twice each year: around 20 March and 22–23 September. In other words, it is the point at which the center of the visible Sun is directly above the Equator.
Daytime and nighttime are never exactly equal, however, due to the angular size of the sun, and atmospheric refraction.
Left: The sun's rays now illuminate almost eaxactly 50% of the Earth at the Equinox. September is the Autumn (Fall) Equinox in the Northern hemisphere and the Vernal (Spring) Equinox in the Southern. They are reveres in March.
Right: