Yom Kippur Katan

It appears to have been inaugurated in the sixteenth century at Safed by the kabbalist Moses Cordovero,[1] who called the fast Yom Kippur Katan; and it was included by Isaac Luria in his Seder ha-Tefillah.

[2] When Rosh Chodesh occurs on Shabbat or Sunday, Yom Kippur Katan is observed on the preceding Thursday.

[3] The custom has roots in scripture (Numbers 28:15) where a sin offering is sacrificed on Rosh Hodesh, indicating judgement and atonement is provided by God on that day.

[5] The selichot are taken partly from the selichos of mincha from Yom Kippur, with the Viddui ha-Gadol (the great confession of sin by Rabbenu Nissim) and Ashamnu, and also a beautiful poem written for the occasion by Leon of Modena and beginning with Yom zeh.

As stated above, if the 29th of the month falls on a Friday or a Sabbath, Yom Kippur Katan is observed on the Thursday prior.

A sign announcing fast day prayers for Yom Kippur Katan in the Zichron Moshe Synagogue in Jerusalem. The special Torah Reading for Mincha of a fast day, as announced on this sign, is done only if at least ten men are fasting.