Every September, the Sun reaches the autumnal equinox — the point in its annual travels at which it's directly over the equator, heading south in the sky. In 2012, this equinox occurs on the 22nd.
While that's going on, the planets Saturn and Mars are bidding us adieu, low in the west. The best time to spot them is about 45 minutes after sunset, as shown in the diagram here.
Meanwhile, Jupiter is gradually moving out of the predawn sky, leaving Venus in complete dominance there, and now rises around midnight. In a few months it will dominate the evening sky.
To get a personally guided tour of these sky sights and others, all visible to the unaided eye, you can download September's 6-minute-long audio sky tour. It's a 6-MB MP3 file. Enjoy!