Zoroastrianism assigns its worship to watches of the day, and one liturgy alone belongs to the deep night. The Vendidad Service begins in the Ushahin gah, after midnight, and continues toward dawn: the two priests, purified and installed in the ritual precinct, perform the Yasna enlarged by the Visperad, and into that already extended service they recite, at fixed stations, the twenty-two chapters of the Vendidad, the law against the demons. The choice of hour is doctrine enacted.
The night belongs, in Zoroastrian cosmology, to the assault of the polluting powers, and the recitation of the law that names, contains and repels them is set precisely where the danger is greatest, so that the priests keep watch for the community with the most powerful instrument the tradition owns, the sacred word itself. The service is physically formidable: six hours and more of continuous Avestan recitation from memory and book, sustained by two men, with the ritual actions of the Yasna proceeding beneath the added text. It has traditionally been performed for the protection of the living, for the souls of the departed, and at seasons when the community's need is great. Because of its length and the strictness of the required priestly purity, the full Vendidad Service has become rare, and its decline is one of the most discussed losses within the contemporary Parsi priesthood; where it is still performed, it remains what it always was, the heaviest single act of Zoroastrian worship.