It's the midsummer Eve's eve. The day most of us Finn's head towards north to the cabins, camping areas or other places to spend the weekend. For the next three days our cities are mostly deserted. It's hard for me to even imagine the midsummer in the city.
Midsummer is the old celebration of the longest day of the year during the summer solstice. Of course the Christianity has assimilated it as many other old mark days and for most it's known as the day of the John the Baptist. In Finland the day is even called Juhannus after him.