Interesting definition, i never thought about it in this
way.
It seems that in Scandinavia the celebrate June 23rd as
Midsummer. <a href=http://www.ingebretsens.com/archive/scan_midsummer/archive_scan_midsummer.htm">Here
they write:
<quote>Midsummer, an observation of the longest day of the year, is
celebrated in each of the Scandinavian countries. Midsummer,
originally a pagan holiday, is celebrated on the day of John the
Baptist and is now held on the weekend closest to the summer
solstice.</quote>
But which date Shakespeare meant, i don't know.