This ol' perennial chestnut.

Both the astronomical and the meteorological calendars are not that good
'fit'.

Spring is well under-way by March 21st with it usually starting around
the 3rd week of February (though there's often a 'touch' of spring even
in mid-winter). I think the 1st of March is better suited for spring's
start.

September 1st can hardly be called the start of autumn especially when
you consider that June on average is no warmer than September despite
the longer days and more sunshine.

If you use nature as a guide then it follows along these lines in most
years:
Winter = December to mid-February
Spring = mid-February to mid-May
Summer = mid-May to end of September
Autumn = October to end of November.

Summer being the longest season with autumn the shortest.

As Len says, big differences depending on where you are in the northern
hemisphere; even quite large differences within the British Isles.

In Orkney if felt rather autumnal by mid-August, generally around 6
weeks earlier than here!

--
Nick Gardner
Otter Valley, Devon
20 m amsl
http://www.ottervalley.co.uk