What is the first prayer service of Yom Kippur eve called?

The first prayer service traditionally associated with Yom Kippur eve is called Kol Nidre.

Kol Nidre takes its name from its opening Aramaic words, meaning “all vows.” Before sunset, worshipers gather, the ark is opened, and the cantor chants the declaration, often three times with increasing intensity. In many communities, the entire evening is popularly called Kol Nidrei night.

Strictly speaking, Kol Nidre is a declaration rather than a prayer service. It is followed by Maariv, the regular evening prayer service, along with additional penitential prayers. The timing can vary: some traditions recite Kol Nidre before sunset, while others begin it at or near nightfall.

Its legal subject is the release or annulment of certain personal vows made to God, not promises made to other people. The melody associated with Kol Nidre became especially famous in Ashkenazi communities, while Italian and Romaniote traditions historically used Hebrew versions of the text.