First day of the season...?

Why do the European and US summer seasons start on June 21st (mid month) when all our seasons start on the 1st of the month?
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In Australia our seasons run like this...
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Winter - June 1 to August 31
Spring - September 1 to November 30
Summer - December 1 to February 28
Autumn - March 1 to May 31
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Just curious...