The 21st of June is the "first day of Summer" and ALSO midsummer's day, it
being the longest day in the year (the Summer solstice).

.. and pretty stupid that sounds too! :-) Unless you're making a
comment about the length of our summer how can the first day of summer
also be "midsummer's day"

Totally agree. I always get annoyed with these pronouncements that
"today is offically the first day of spring/summer/whatever". Seasons
vary across the country. Spring comes much earlier in the Scilly Isles
than it does in Norfolk, I can tell you! Why do we *need* to identify
the seasons by dates. Seasons are natural, dates are man-made.