Kol Nidre (all vows) is the Aramaic declaration chanted at the opening of the Day of Atonement (Yom Kippur), before nightfall, and it gives its name to that evening service. In legal form it is not a prayer but a formula annulling in advance the vows and oaths of a personal, religious kind that one may make and fail to keep in the year to come (or, in another version, the year past), so that the worshipper enters the day of judgement unburdened by broken promises to God. Because its wording seemed to make light of oaths, it was long controversial, criticised from within Judaism and seized upon by hostile outsiders, and its precise legal sense was much debated; yet it was never abandoned. Its extraordinary hold owes less to its legal content than to the solemn, haunting melody to which it is chanted three times as the most sacred day of the year begins, a melody that has become one of the most recognisable in all of Jewish worship.
One historically influential, though now largely rejected, theory held that the formula's roots lay in the need of Jews forcibly converted to Christianity in medieval Spain and Portugal to annul vows of conversion taken under duress, giving the prayer an association with crypto-Jewish survival that still colors its popular reputation even though most scholars now date its core wording earlier than that period. The tune most widely used today, in a minor key with a distinctive rising phrase, is a relatively late traditional melody rather than an ancient one, and it has inspired concert and orchestral arrangements far beyond the synagogue.