Autumn Equinox September 22, 2013

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

Fall equinox tomorrow 9-22-13

Sunday, September 22, 2013 at 1:44 PM PDT

Why is it called equinox?

On the equinox, night and day are nearly exactly the same length – 12 hours – all over the world. This is the reason it's called an "equinox", derived from Latin, meaning "equal night".
However, even if this is widely accepted, it isn't entirely true. In reality equinoxes don't have exactly 12 hours of daylight
The September equinox occurs the moment the sun crosses the celestial equator – the imaginary line in the sky above the Earth’s equator – from north to south.
This happens either on September 22, 23, or 24 every year. On any other day of the year, the Earth's axis tilts a little away from or towards the Sun. But on the two equinoxes, the Earth's axis tilts neither away from nor towards the Sun, like the illustration shows.

Autumnal Equinox Today 9-22-12

Fall for 2012 begins in the Northern Hemisphere on September 22, 10:49 A.M. EDT. Here’s more about the first day of fall—the autumnal equinox—plus, facts, folklore, and some fantastic fall photos! The word equinox comes from the Latin words for "equal night." The fall and spring equinoxes are the only days of the year in which the Sun crosses the celestial equator. From here on out, the temperatures begin to drop and the days start to get shorter than the nights.

Source Farmers Almanac