According to the website USA.gov, the first day of Fall (Autumn) is Wednesday September 22, 2010 at 11:09 PM ET.
Even though most calenders mark the first day of fall as September 23rd, the actual autumnal equinox will take place at approximately 11:09 AM ET tomorrow, the 22nd. Even though Fall is occurring everywhere in the Northern hemisphere, it of course depends on where you live whether or not you notice much difference. Areas like Florida, and the Western US will not notice as much change as northern areas such as Canada, Alaska, and Scandinavia.
Happy Fall Everyone!