Depending on where you are in the Northern Hemisphere, it is the longest day of the year. Actual dates of celebrations vary in countries, often a weekend nearest the true solstice for convenience. It is mostly a northern European festival with pagan roots like so many of their traditional holidays.
"Juhannus" in Finland became so called when Christians renamed it St. John's Eve. I fondly remember the wonderful looong days and very short almost mere twilit nights, some call "white nights" of the North, with bonfires on the beaches and much merriment. It's a very important celebration even for emigrant Scandinavian communities in Canada and other parts.
So, Hauskaa Juhannusta! Happy Midsummer!
Addendum June 24.04: Torill has written a magical story of a Norwegian midsummer night - 'Dreams in the shortest night'.
And Solstice at Stonehenge via BBC