Formally known as Swedish Flag Day until 1983, the holiday commemorates the ascension of King Gustav Vasa to the Swedish throne on the 6th of June, 1523. This was following the Swedish War of Liberation (1521 - 1523) against Denmark, which also saw the dissolution of the Kalmar Union (a personal union of Demark, Norway, and Sweden where the Danes held the most power).
First observed in 1916 at the Stockholm Olympic Stadium, it didn't become a full-fledged holiday until decades later, before finally become an offiial public holiday in 2005.
One notable thing is the King of Sweden inviting and allowing the general public to tour parts of the Stockholm Palace for free on this day.
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As for the pic, I can't vouch for its accuracy. Perhaps our resident Swede can render a proper analysis.
First observed in 1916 at the Stockholm Olympic Stadium, it didn't become a full-fledged holiday until decades later, before finally become an offiial public holiday in 2005.
One notable thing is the King of Sweden inviting and allowing the general public to tour parts of the Stockholm Palace for free on this day.
*
As for the pic, I can't vouch for its accuracy. Perhaps our resident Swede can render a proper analysis.