Sabbaths are celebratory feasts where witch covens gather, feast, and perform rituals.
Candlemas (February 2). Witches make their own black candles. Tallow wax always mixed with human blood. Many contain poisonous herbs too. Some candles have specific purpose. Can make victims fall helpless into deep sleep. Or become in thrall to a witch. Also can make time seem to pass more quickly or slowly than it really does.
Walpurgis Night (April 30). Sabbath when novitiates are assigned to the witches who'll train them Blood rituals. Spells of binding. Attempts to control trainees and take away free will. Not all survive. Blood of the slain used to anoint the successful.
Lammas (August 1). Most propitious sabbath for opening portal to the dark. This is when attempts made to contact the Fiend or summon him into our world. Needs at least combined strength of three covens to attempt this safely.
Halloween (October 31). Celebrates coming of the winter months. Witch feast when ghosts most receptive to interrogation. Forced to answer questions and some can prophesy. Witches usually wear masks at this sabbath. Even when main clans celebrate separately, spies will infiltrate. If detected spies slain, their blood and bone taken for dark magic.
Source: The Journal of Thomas J. Ward (Book 4, Attack of the Fiend)