Walpurgis Night (Valborgsmässoafton in Swedish, Vappu in Finnish, Volbriöö in Estonian, Valpurģu nakts or Valpurģi in Latvian, Walpurgisnacht in German), Čarodějnice in Czech is a holiday celebrated on April 30 or May 1, in Finland, Sweden, Bohemia (Czech Republic), Estonia, Latvia and Germany.
The festival is named after Saint Walburga (known in Scandinavia as "Valborg"; alternative forms are "Walpurgis", "Wealdburg", or "Valderburger"), born in Wessex in 710 a niece of Saint Boniface. According to legend, she was a daughter to the Saxon prince St. Richard. Together with her brothers she travelled to Württemberg, Germany where she became a nun and lived in the convent of Heidenheim, which was founded by her brother Wunibald. Walburga died on 25 February 779 and that day still carries her name in the Catholic calendar. However she was not made a saint until 1 May in the same year, and that day carries her name in the Swedish calendar.
Historically the Walpurgisnacht is derived from Pagan spring customs, where the arrival of spring was celebrated with bonfires at night. Viking fertility celebrations took place around April 30 and due to Walburga being declared a saint at that time of year, her name became associated with the celebrations. Walburga was worshipped in the same way that Vikings had celebrated spring and as they spread throughout Europe, the two dates became mixed together and created the Walpurgis Night celebration.
In Germany, Walpurgisnacht, the night from April 30 to May 1, is the night when allegedly the witches hold a large celebration on the Blocksberg and await the arrival of Spring. When the travel to that mountain is too far, they may also chose another hill in the region they live.
"Walpurgis Night (in German folklore) the night of April 30 (May Day's eve), when witches meet on the Brocken mountain and hold revels with their Gods..."
"Brocken the highest of the Harz Mountains of north central Germany. It is noted for the phenomenon of the Brocken spectre and for witches' revels which reputably took place there on Walpurgis night. The Brocken Spectre is a magnified shadow of an observer, typically surrounded by rainbow-like bands, thrown onto a bank of cloud in high mountain areas when the sun is low. The phenomenon was first reported on the Brocken."
—Taken from Oxford Phrase & Fable.