1820, from German Walpurgisnacht, witches' revel, especially on the Brocken in the Harz Mountains, on May-day eve, literally "the night of (St.) Walpurgis," from Walburga, English abbess who migrated to Heidenheim, Germany, and died there c. 780; May 1 being the day of the removal of her bones from Heidenheim to Eichstädt.
late 14c., "tendency to change, inconstancy," from Old French mutabilité, from Latin mutabilitas, from mutabilis "changeable" (see mutable).
It is the same!—For, be it joy or sorrow,
The path of its departure still is free;
Man's yesterday may ne'er be like his morrow;
Nought may endure but Mutability.
[Shelley, from "Mutability," 1816]