Astronomically, it begins with the autumnal equinox (around September 23 in the Northern hemisphere, and March 21 in the southern hemisphere), and ends with the winter solstice (around December 21 in the Northern hemisphere and June 21 in the Southern hemisphere). However, meteorologists count the entire months of March, April and May in the Southern hemisphere, and September, October and November in the Northern hemisphere as autumn.
http://uk.answers.yahoo.com/question...4033531AAtQVKb