When is the September equinox and what happens?

Far from being an arbitrary indicator of the changing seasons, March 20 (March 21 in some years) is significant for astronomical reasons. On March 20, 2008, at precisely 1:48 A.M. EDT (March 20, 05:48 Universal Time), the Sun will cross directly over the Earth's equator. This moment is known as the vernal equinox in the Northern Hemisphere. For the Southern Hemisphere, this is the moment of the autumnal equinox.

This also happens on September 22/23

The days and nights are of equal length