Sankt Hans aften (St. John's Eve) was an official holiday until 1770. Typical of Danish traditions, the holiday is celebrated on the evening before the actual day. It takes place on the evening of 23 June. It is the day where the medieval wise men and women would gather special herbs that they needed for the rest of the year to cure people.
It has been celebrated since the times of the Vikings by visiting healing water wells and making a large bonfire to ward away evil spirits. Today the water well tradition is gone. Bonfires on the beach, speeches, picnics and songs are traditional, although bonfires are built in many other places where beaches may not be close by (i.e. on the shores of lakes and other waterways, parks, etc.) In the 1920s a tradition of putting a witch made of straw and cloth (probably made by the elder women of the family) on the bonfire emerged as a remembrance of the church's witch burnings from 1540 to 1693. This burning sends the "witch" away to Bloksbjerg, the mountain 'Brocken' in the Harz region of Germany where the great witch gathering was thought to be held on this day.
Holger Drachmann and P. E. Lange-Müller wrote a midsommervise (Midsummer hymn) in 1885 called "Vi elsker vort land..." ("We Love Our Country") that is sung at every bonfire on this evening. From what I saw, Danes mumble their way through this (even though they had song sheets), and the language has changed since the song was written - so some words are hard to translate.
Either way, it's a fun tradition. It's the longest day (like really long - they sun set after 11.30).
Though I did feel sorry for the mock witch. The one at Frederiksberg had screaming fireworks in that didn't go pop-bang-oh-ah, but just screamed. Poor withchey!
P.S. I flogged most the text rom wikipedia!
