Walpurgis combines the Old German walh (foreign) and burg (protector) — "strong protector." Saint Walpurga (c. 710-779) was the English-born abbess of Heidenheim in Germany — her feast night, Walpurgisnacht (April 30), is the famous German witches' night.
Walpurgisnacht features in Goethe's Faust.
Walpurgis reduces to three — the number of German witches' night.