June 21st is the first day of summer, as it's a astronomical measure of
time.

I've never quite got that though. If astronomical summer is defined as
the lightest season (which, surely, it ought to be, just like climatic
summer is the warmest season) then astronomically, summer should be the
three months centred on June 21st, i.e. May 6th to August 6th.

A combination of day length, temperature, and relative probability of
sun vs gloomy overcast would be something like this:

Summer: June 1 - September 20
Autumn: September 21-November 20
Winter: November 20-February 28
Spring: March 1 - May 31

Autumn in particular is a fleeting season: high daytime temps (20C+)
typically last well into September in southern England whilst in a
normal year most of the leaves have dropped off by mid-November,
leaving us in the dank gloom of winter.

Nick