Tonight is Walpurgis Night. Saint Walpurga was, supposedly, an 8th century nun whose relics were transferred on April 30th and her name was given to the holiday. As frequently happened during Christianity's spread across Europe, this was merely an instance of co-opting and Christianizing a much older holiday.
Before Saint Walpurga was dreamed up, 30 April was known as day that Odin died attempting to retrieve the knowledge of the runes and, ever since, the barrier between the world of the dead and the living is at its weakest on this night. For this reason, Odin's faithful build bonfires, play drums and (nowadays) shoot off guns tonight to ward off evil. In some German-speaking areas, tonight is known as Hexxenacht because on this night witches are said to mount their broomsticks and fly from near and far to Brocken Mountain where they "hold revels with their Gods."
In rural Bavaria, children sometimes play pranks-- wreaking destruction on gardens, TP-ing homes, smearing toothpaste in places unintended by the manufacturer, and writing graffiti. In the Rhineland boys bring trees hung with colored streamers to the houses of girls they like. A tree decorated only with white streamers indicates dislike for the recipient. In southern Sweden children gather greenery at twilight and adorn their homes. In Finland, they drink sparkling wine and sima (a mead) and burn strawmen imbued with ill-will and bad luck. In Estonia and Germany, girls dress as witches and make merry.