Yom Kippur Katan
A fast day associated with the new moon.
Excerpted with permission from The Jewish Religion: A Companion, Oxford University Press.
"Minor Yom Kippur" [is] the name given to the day before Rosh Chodesh ("New Moon"), in that this day is treated as one of fasting, repentance, and supplication on the analogy of Yom Kippur. Yom Kippur Katan originated among the Safed Kabbalists in the 16th century and is referred to by a disciple of Moses Cordovero, Abraham Galante, who states that it was a local custom in Safed for men, women, and schoolchildren to fast on this day and to spend the whole day in penitential prayer, confession of sin, and flagellation.
There is no reference to Yom Kippur Katan in the standard Code of Jewish law, the Shulchan Arukh, but a later Halakhist, Joel Sirkes, in his commentary to Jacob ben Asher's Tur, mentions it and as a result the day acquired something of a Halakhic footing and came to be observed in communities with little connection to the Kabbalah. A number of small booklets were published containing the prayers and customs of the day. Nowadays, Yom Kippur Katan has largely fallen into disuse, yet the rite itself is of interest for its amalgam of Talmudic and Kabbalistic themes.
The Talmud (Hullin 60b) quotes an amazing comment of Rabbi Simeon ben Lakish that the he‑goat offered on Rosh Chodesh is called "a sin offering unto the Lord" because it is an atonement for God Himself for having made the moon smaller than the sun. Arising out of this is the idea, expressed in the Rosh Chodesh liturgy, that Rosh Chodesh affords pardon for Israel's sins.