"The wind is hushed, the starlight pales,
The dismal moon her features veils;
As magic-mad the hosts whiz by,
A myriad sparks spurt forth and fly."
(from Goethe's Faust)
Ah, Walpurgisnacht! (Hexennacht in Germany, Valborgsmässoaften in Sweden). That supernatural, vernal equivalent to Halloween, when spirits fly abroad and the witches (so they say) go to dance on Mt. Brocken (immortalized in film in Disney's Fantasia).
Jacob Grimm (yeah, the same guy who wrote the fairy tales with his brother), in his book Teutonic Mythology, described it thus:
"Down into the tenth and into the 14th centuries, night-women in the service of Dame Holda rove through the air on appointed nights, mounted on beasts; her they obey, to her they sacrifice, and all the while not a word about any league with the Devil. Nay, these night-women, shining mothers, dominae nocturnae, bonnes dames ... were originally daemonic elvish beings, who appeared in woman's shape and did men kindnesses. Holda, Abundia, to whom still a third part of the whole world is subject, leads the ring of dancers ... "
Well, even if you don't believe in witches, it's still a good excuse for a party! (Just be careful who you dance with!)