Should you fast or daven the special prayers for Yom Kippur Katan? I have never done this in the past, but my son, in yeshiva, urges me to do so.
What is the status of these customs?
Yom Kippur Katan, which is observed by fasting or by reciting special prayers on the eve of Rosh Chodesh, is a worthy practice.
However, it is not obligatory, and many do not commemorate the day.
The custom of observing Yom Kippur Katan (by fasting) is of comparatively recent origin. It is not mentioned in the Shulchan Aruch or by other early authorities, and appears to have been inaugurated in the sixteenth century by Rabbi Moshe Cordovero (see Peri Chadash, Rosh Ḥodesh, no. 417).
The fast was introduced by the Arizal into the prayer service, and the Shelah refers to it as the Yom Kippur Katan service, and writes that it should be observed by fasting and repentance: “Following the custom of the very pious, one must repent of his ways and make restitutions both in money and in personal acts, in order that he may enter the new month as pure as a new-born infant.”
When Rosh Chodesh occurs on Shabbos or Sunday, Yom Kippur Ḳaṭan is observed on the preceding Thursday.
The custom has roots in the Torah and in Chazal. The pasuk (Bamidbar 28:15) writes that a Sin Offering is ordered for Rosh Chodesh, which highlights an issue of judgment and atonement. Because it is forbidden to fast on Rosh Chodesh itself, it became customary for the pious to fast on the day prior to Rosh Chodesh.
The Gemara (Chullin 60b) writes, citing from Rabbi Shimon b. Lakish, that the sa’ir offered on Rosh Chodesh is called “a sin offering unto Hashem” because it is an atonement for Hashem for having made the moon smaller than the sun. Arising out of this is the idea that Rosh Chodesh affords pardon for Israel’s sins.
Yom Kippur Katan is not observed for the following months: Cheshvan (because Yom Kippur has just passed); Teves (because it would fall during Chanukah when fasting and penitential prayers are not permitted); Iyar (because it would fall during Nissan which doesn’t allow fasting); and Tishrei (because it would fall on the day of Erev Rosh Hashanah which doesn’t permit penitential prayers).
The custom is noted by later authorities, including the Mishnah Berurah (417), who writes that some have the custom of fasting on the eve of Rosh Chodesh, and expounds a little on the different customs involved. He concludes by stating that even those who don’t fast should make it their practice to repent and reflect on their deeds on this day, so that Rosh Chodesh will indeed be a day of atonement for them.