Seasons of 2011:
SPRING EQUINOX March 20, 7:21 P.M. EDT
SUMMER SOLSTICE June 21, 1:16 P.M. EDT
FALL EQUINOX September 23, 5:05 A.M. EDT (Fall's Beginning)
WINTER SOLSTICE December 22, 12:30 A.M. EST (Fall's End)
Source:
Almanac
First Day of Autumn
Fall Equinox - day when night and day are of equal length
September (9/22/08, 9/22/09, 9/23/10,
9/23/11, 9/22/12, 9/22/13, 9/23/14, 9/23/15, 9/22/16, 9/22/17, 9/23/18, 9/23/19, 9/22/20)
First Day of Winter [a.k.a. Last Day of Autumn]
Winter Solstice - shortest day of the year
December (12/21/08, 12/21/09, 12/21/10,
12/22/11, 12/21/12, 12/21/13, 12/21/14, 12/22/15, 12/21/16, 12/21/17, 12/21/18, 12/22/19, 12/21/20)
Source:
HolydaySmart
Also, North America's Fall coincide with Brazilian Spring (I'm Brazilian), which goes from Sept 23th to Dec 21th or Dec 22th.