In article , Jo Ling
writes
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).

Who came up with the 24th of June? That's the feast of St Johann, if I
remember rightly, and now to do with being the first, last or middle day of
Summer.

I think it is St John the Baptist's day. I have no idea why it is
called midsummer's day.
--
June Hughes