See Wikipedia and Swiss World to see more information about this holiday which is like the American 4th of July.
Swiss World says:
Long before the government decided in 1891 to declare the day Switzerland's national day, people had celebrated summer by lighting bonfires. Indeed, the custom of lighting a fire on June 24th, St John's Day, is known all over Europe. But for the Swiss, bonfires had an extra significance. For centuries they had built beacons on mountain tops which they lit when danger approached. One legend told of both Lake Geneva and Lake Biel relates how hordes of invading barbarians intent on conquering the ancient Swiss tribes turned back when they saw the lights reflected in the lake waters, thinking they had come to the edge of the earth and were about to ride off into the sky.
Whether in remembrance of this event, or just because it is fun, every Swiss commune now lights its own bonfire and sets off fireworks, and children parade through the streets with paper lanterns - often decorated with the Swiss cross or the symbols of the cantons - and people light candles in their windows.
