when does autumn start??

Astronomically, some Western countries consider autumn to begin with the September equinox (around September 23) in the northern hemisphere, and the March equinox (March 21) in the southern hemisphere, ending with the December solstice (around December 21) in the northern hemisphere and the June solstice (June 21) in the southern hemisphere.[1] Such conventions are by no means universal, however. An example is found in the Irish Calendar which still follows the Celtic cycle, where autumn is counted as the whole months of August, September and October. In Chinese astronomy, the autumnal equinox marks the middle of autumn, which is deemed to have begun around the time of Liqiu (around August 7).

On the other hand, 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