Walpurgis Night, in Germanic lore, is the night before May Day, when the witch-world holds its most elaborate revels, particularly (according to Goethe) at The Brocken, high-point of the Harz Mountains, in the presence of the Devil. It's the start of Summertime, Hallowe'en reflected six months across the calendar.
The name 'Walpurgis' derives from a Christian nun, Walburga (known in Scandinavia as 'Valborg'). She was born in eighth-century Wessex, but latterly controlled the monastery and nunnery at Heidenheim in Germany (which was founded by her brother St Willibald; her other brother, St Winebald, had been the first abbot there).
St Walburga's bodily remains were transferred to Eichstätt on a May Day, and that's how an English nun's memory got entwined with the pagan celebrations of Beltane, the Celtic quarter-day, May 1st.
More Walpurgis revels here