A supposed meeting of those who practice witchcraft and other rites.
In Europe during the Burning Times of the Early Modern Period, the mass paranoia of the times fed the belief that witches, at special times of the year, flew to these secret meetings/ festivities held in remote areas, typically in the forest or in the mountains (places like Brocken or Bald Mountain). The popular imagination envisioned a secret society that turned every moral norm of mainstream society on its head.
At the witches sabbath, Satan was supposed to have presided over the congregation and initiated new witches in a face-to-face pact. The Sabbath was imagined to begin with a mockery of Christian rites and "baptism" with new satanic names, building into an orgy of naked dancing, sex with demons (including Satan himself), and gluttonous feasting on the flesh of human infants.