(in German folklore) the night of April 30 (May Day's eve), when witches meet on the Brocken mountain and hold revels with the Devil. It is named after St Walburga, an English nun who in the 8th century helped to convert the Germans to Christianity; her feast day coincided with an ancient pagan festival whose rites were intended to give protection from witchcraft.
From The Oxford Dictionary of Phrase and Fable
