The Autumnal Equinox takes place at 4:44 PM this Sunday, September 22. It marks the instant that the Sun is directly over the Earth's equator. At this point, the period of day and night are equal - this is where the term equinox comes from.
The Sun has been moving south since last June 21 when the Sun was as far north as it gets. On that day, it was overhead of latitude 23.5 degrees north and summer began in the northern hemisphere. Over the next few weeks, note the position along the western horizon where the Sun sets. Notice how quickly the position shifts to the south. Three months from now, on December 21, the south will set at its furthest southern point and winter will begin. Brrrrr.
The impact of this north-south motion of the Sun? The Seasons!
Read about the equinox in this article from Sky and Telescope magazine.