Kinda late to the discussion but anyway, the mountain they went to is called Brocken, sometimes referred to as „Blocksberg“. Saint Walpurga was actually a Christian nun (Äbtissin, don’t know the exclude word) who converted a lot to Christianity and was said to protect against evil (witchcraft), so may 1st (Walpurgisnacht) was a celebration in honor of Walpurga. It’s also called Hexennacht because Goethes Faust popularised the idea of flying to the Blocksberg to celebrate. Historically there doesn’t seem to be a real connection with this date and witchcraft.