Rabbi Leone Da Modena and the custom of Yom Kippur Katan
Yom Kippur Katan,literally, "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 similar to 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.
Yom Kippur Katan is very interesting for a number of reasons.
Its main piyut (liturgical poem) yom ze yehi mishkal kol chatotai, was composed by a vociferous opponent of Kabballa, namely the 17th century Italian Rabbi, Leone(Yehuda Aryeh)Da Modena (1571-1648). Rabbi Da Modena was a fascinating and highly colorful figure, one of the few early Rabbis who wrote an autobiography (and a fascinating one at that, highly candid and often emotional, which I urge everyone to read, particularly the annotated english translation by Mark Cohen).
Rabbi Da Modena also authored a classic anti-Kabbalistic tract called Ari Noham and is considered to be one of the most outspoken opponents of Lurianic Kabballah and Zohar. It is interesting and highly ironic that he is so tied in with a custom that has its origins in Kabballah. I often wonder what the people who say this poem would think, if they knew who the author was.
There is an additional interesting paradoxical twist to this ritual. A friend pointed out to me that this is one of two customs-clearly based on the Kabballah- that was universally accepted and held in high regard by the true Ashkenazic kehilot (Germany, Bohemia, Moravia and Hungary) which were generally wary (to say the least) of practices based on Kabballah. The other one is saying the Tikkun Leil Shavuot as established by the Kabbalists.
He also pointed out that apparently there were indeed several Charedi Rabbis who felt that the aforementioned piyut (yom zeh) should not be said, due to the controversies surrounding its author. The most prominent of these was Rabbi Dovid Jungreis, of the Edah Hacharedit of Jerusalem .
More on Rabbi Da Modena in upcoming posts. (why did Shadal dislike him? Why was he bareheaded? is it true that he changed his views on gilgul toward the end of his life, and what prompted him to do so? and more).
 see Minhogei haKehilot, Volume 1 under the customs of Rosh Chodesh.