Walpurgis Night (
Walpurgisnacht in German) is "the other Hallowe'en."
It's six month from All Hallows Eve, the day before All Saints' Day. Much less famous (infamous?) than trick or treat night in the U.S., Walpurgis Night falls on the eve of May Day.
From ancient pagan beliefs adopted by early Christians, it's also a night when witches or demon spirits are on the loose till midnight, and then are chased away by celebrations of spring.
Walpurgis Night - The Other Halloween