It is a celebration (originally pagan) of the coming of spring, celebrated mostly in Germany and Scandinavia. It is much like Halloween, when the change of season is believed to make communication with the dead possible. It was later Christianized in honor of Saint Walpurga.
https://www.vam.ac.uk/blog/museum-life/walpurgis-night-the-witches-sabbath-and-the-first-of-may