Walpurgisnacht, or Walpurgis' Night is not the vernal equinox (the vernal equinox, also Ostara, is around March 21st, when the sun is entering the constellation
Aries from earth's perspective). Walpurgis' Night is the evening of April 31st, which leads to
May 1st, and is around the
time of the Celtic Beltane (From an astronomer or astrologer's perspective, the sun will be around the 15th degree of Taurus from earth's perspective, or simply the middle portion of Taurus. It is also the 2nd decan of Taurus). As a
key note, Beltane is related to fertility rituals (after all, it is the spring) and may also be known as May
Day.
The name "Walpurgis" is referring to St. Walburga, who was an educated
English abess that studied Latin (remember, educated women were rare back then), and migrated to Heidenheim, Germany, and died there around 780 CE. May 1st is also the
day of the removal of her bones from Heidenheim to Eichstädt.
The witchy connotations of Walpurgis' Night may have sprung up around the late 18th century and early 19th century CE in Germany, and memorialized when Goethe published his play about the
legend of Faust in 1808. It is possible that Walpurgis' Night may have been associated with witchcraft due to the pagan's religious practices around this time in accordance with May
Day. Coupled with the
church's demonization of pagan practices, these religious practices around this
time was marked as witchcraft, hence leaving Walpurgis' Night with a witchy association.
Walpurgisnacht, as seen in Puella Magi Madoka Magica, is one of the many references in the
anime to the Germanic Faustian
play by
Johann Wolfgang von Goethe, Goethe's Faust.