Summer solstice is on June 21st, but due to traditional calendar confusion, we celebrate the event on the eve of June 23rd. For most people (probably more or less everyone) this is just a traditional party day. But in former times this was the most dangerous night of the summer season, just like Christmas Eve in the winter. All the evil forces were loose, and people had to do whatever they could to protect themselves. Houses, entrances at least, were decorated with foliage to keep the evil forces out, and huge bonfires were lit during the short night for the same reason. The witches had their traditional gathering on Midsummer's Eve.

When the new religion was forced upon our ancestors around a thousand years ago, the new rulers were smart enough to realize that the pagan traditions of midsummer were far too strong to be ignored. Midsummer was immediately made a christian celebration in honour of St. John the Baptist. St. John in Norwegian is St. Johannes, and the name of both the man and the holiday was shortened to St. Hans, which is still what we call the celebration: St. Hans' Eve. The only difference in the tradition after the new religion took over, was the fact that now also the churches were decorated with foliage and that people tended to take refuge inside the churches for the rest of the night after the last flames of the bonfires had died out.
Today, here on the south coast, where everyone seem to have at least one boat of their own, the celebration starts with the crowding of boats in Grimstad harbour around 6 PM. The foliage has been moved from the houses to the boats, but the bonfires are still burning as soon as it's dark enough to actually see them, around 11 PM. The largest bonfire I've heard of this year, on the northwest coast of Norway, was 39 meters high.
The tradition of summer solstice celebration is still very much alive; the origins very much forgotten.