12. Many people follow the custom of fasting during the "Ten Days of
Repentance" ("Aseres Ye'mei T'shuvah) (1). Since there are four days of
these ten on which it is improper to fast, that is, the two days of Rosh
HaShanah, Shabbos, and the day preceding Yom Kippur, the people who follow
that custom, fast on four other days during the period of Selichos before
Rosh Hashana (2), in place of [those missing four days]. [It is customary
to fast] on the first day of Selichos, the day preceding Rosh HaShanah, and
two other days [during the Selichos period, preferably] a Monday and a
Thursday. If one is invited to participate in a feast associated with a
mitzvah [on one of these four days], one may do so and fast on another day
instead. If he knows that he will participate in such a feast, he should
fast the day before [the meal], to make up for it.
(1) These are the ten days from the first day of Rosh Hashana up until and
including Yom Kippur (that is, from the first of Tishrei until the tenth).
According to the Talmud, our prayers and repentance are accepted much more
readily during these ten days (we'll see more about this in Chapter 130).
(2) Selichos are recited for a number of days before Rosh Hashana
(depending on which day of the week Rosh Hashana occurs), and each morning
throughout the "Ten days of Repentance."