There are two equinoxes every year – in September and March – when the sun shines directly on the equator and the length of day and night is nearly equal. Seasons are opposite on either side of the equator, so the equinox in September is also known as the "autumnal (fall) equinox" in the northern hemisphere. However, in the southern hemisphere, it's known as the "spring (vernal) equinox".
The Earth's postion during the September equinox (ill. not to scale).
September Equinox in Angels Camp, California, U.S.A. is on Monday, September 22, 2014 at 7:29 PM PDT