The German motto "Tanz in den Mai!" (Dance into May!) derives from a tradition called Walpurgisnacht. The Walpurgis celebrations, which are of Pagan origin, are traditionally held the night before May Day in the Harz Mountains in Germany, but also across much of northern Europe. Revellers light bonfires, dance around naked and stay up until dawn. For those of us who won't be on the Continent this weekend, there will be a Walpurgis Night rave on London's Hampstead Heath on Saturday.