Kol Nidre is a Hebrew and Aramaic declaration which is recited in the synagogue before the beginning of the evening service on the night of Yom Kippur. Strictly speaking, it is not a prayer, even though it is commonly spoken of as if it were a prayer. This declaration and its ceremonial accompaniment have been charged with emotional undertones since the medieval period, creating a dramatic introduction to Yom Kippur on what is often dubbed "Kol Nidrei night", with the entire Yom Kippur evening service popularly called Kol Nidrei.

Yom Kippur is the holiest day of the year in Judaism and Samaritanism. It occurs annually on the 10th of Tishrei, corresponding to a date in late September or early October.