The 30th of April is Walpurgis Night in Sweden and in some other
countries as well.
Some say that the bonfires in the past were made to throw out the old (like winter) and welcome the new (spring). Or to scare away the evil spirits and witches...
More info here.
Anyhow, it was a nice gathering and the kids loved it!
Later at night the organizers gave a great firework show!