YOM KIPPUR KATAN (Heb. יוֹם כִּפּוּר קָטָן; lit. "minor day of atonement"), the eve of the new month which became for the pious a day of fast and repentance. The custom of keeping Yom Kippur Katan is a late one, and is not mentioned in the Shulḥan Arukh. It began among the kabbalists of *Safed in the second half of the 16th century and is first spoken of by Moses *Cordovero. The waning of the moon was conceived by the kabbalists as a symbol of the exile of the Shekhinah ("Divine Presence") and the diminution of the power of holiness during the Exile, and its renewal as a symbol of the return to perfection in the age of Redemption. They based this conception on the talmudic legend according to which God had said to Israel: "Bring atonement upon me for making the moon smaller" (Ḥul. 60b). In addition to the reading of the Torah and other prayers and seliḥot, customary for a fast day, special seliḥot were written for the afternoon prayer (Minḥah) of Yom Kippur Katan. They are based on the themes of Exile and Redemption. The special service Tikkun Yom Kippur Katan was first printed in Sha'arei Ẓiyyon (Prague, 1662) by Nathan Nata *Hannover. Later it appeared in different versions and in special books which were very popular until the 19th century. The tikkun (special prayer) in *Ḥemdat Yamin is particularly well known. The first halakhic reference to Yom Kippur Katan appears in Bayit Ḥadash by Joel *Sirkes. The celebration of Yom Kippur Katan became widespread because of the many commendations by Isaiah b. Abraham *Horowitz in Shenei Luḥot ha-Berit. Later the custom became popular among the pious who observed this day as though it were sanctioned by halakhah without any connection with *Kabbalah.
G. Scholem, On the Kabbalah and Its Symbolism (1965), 151–3; idem, in: Beḥinot be-Vikkoret ha-Sifrut, 8 (1955), 93–94; A. Yaari, in: KS, 38 (1962/63), 99, 249–50; A. Abeles, Der kleine Versoehnungstag (1911).