Erev Rosh Chodesh-Yom Kippur Katan:
B. Fasting and saying Selichos:
Fasting: Some pious Jews have the custom to fast on Erev Rosh Chodesh. This fast has become widespread in the later generations. Those who fast are not to delay eating once the night of Rosh Chodesh begins. Thus, it is forbidden to continue to fast past nightfall. This applies even if he did not yet Daven Maariv. Some Poskim rule one is to stop fasting starting from sunset. Some fast only until midday. [Practically, it is no longer the widespread custom even amongst the pious to fast on Erev Rosh Chodesh. It is likewise not the Chabad custom to fast on Erev Rosh Chodesh. One who nevertheless desires to fast is to redeem the fast with charity.]
Selichos and Seder Yom Kippur Katan: Many have the custom to recite Selichos and Seder Yom Kippur Katan [even if they are not fasting on this day]. [Practically, many today are not accustomed to recite the Seder Yom Kippur Katan on Erev Rosh Chodesh. Likewise, it is not the Chabad custom to recite Selichos or Seder Yom Kippur Katan on Erev Rosh Chodesh.] Those that recite Selichos on Erev Rosh Chodesh are to recite it before Mincha. However the custom has become to say it after Mincha.
Erev Rosh Chodesh Elul: The above customs apply particularly to Erev Rosh Chodesh Elul in which it is the custom of many to fast and say the prayers of Yom Kippur Katan even if they are not particular to do so the rest of the year. However, some meticulous men are not accustomed to do so even on Erev Rosh Chodesh Elul, [and so is the Chabad custom].
C. A day of Teshuvah:
The day of Erev Rosh Chodesh is a day of Teshuvah in which one repents for his sins of that month. Thus, even those that are not accustomed to fast [or say Selichos] on this day are nevertheless to spend the day in doing Teshuvah and fixing the sins of the previous month. Certainly one who fasts [or says Selichos] on this day is to mainly emphasize the performance of Teshuvah.
The Mittler Rebbe’s directives:
Erev Rosh Chodesh is an auspicious time for a Baal Teshuvah to be aroused and return to Hashem from the sins of his youth with a broken and shattered heart. Every Erev Rosh Chodesh, towards the prayer of Mincha, he should say after his prayer of Shemoneh Esrei the confession prayer of Al Cheit normally recited on Yom Kippur. Upon saying the words “Al Cheit Shechatanu Lefanecha” he is to remember of all of his past sins and verbalize them before His creator, expressing regret for the past and resolution for a better future. He is to cry from the depths of his heart until his face is filled with tears.
Some Poskim rule it is forbidden to eulogize or say Tziduk Hadin and Kaddish on Erev Rosh Chodesh starting from midday. Other Poskim however rule it is permitted to eulogize and say Tziduk Hadin on Erev Rosh Chodesh even past midday.
Tachanun is omitted starting from Mincha of Erev Rosh Chodesh.
Tzidkasecha: When Rosh Chodesh falls on Sunday, Tzidkasecha is not recited during the Mincha prayer on Shabbos.
F. Haftorah of Shabbos Erev Rosh Chodesh:
If Erev Rosh Chodesh falls on Shabbos, the Haftorah of “Machar Chodesh” is read in place of the weekly Haftorah.
Parshas HaChodesh: If the Shabbos of Parshas HaChodesh is Erev Rosh Chodesh, the Haftorah of Parshas HaChodesh is read. This reading is then followed by the first and last verse of Machar Chodesh.
Shnayim Mikra: On a Shabbos that there are two Haftoras for that week, such as the Shabbos of Erev Rosh Chodesh, one is to read both the Haftorah of the weekly Parsha and the Haftorah which will be read in Shul.
What is the law if the weekly Haftorah was read instead of the Haftorah of Rosh Chodesh?
See Chapter 3 Halacha 11C in Q&A!
Some are accustomed to perform Hataras Nedarim on Erev Rosh Chodesh.
One is to add in Torah, Avoda and Tzedaka on this day.
It is an auspicious time to visit the gravesite of Tzaddikim on Erev Rosh Chodesh and on the 15th of each month. These are the main times that one is to visit the tomb of Tzaddikim.
The idea of Erev Rosh Chodesh being a special day is recorded in the early Acharonim of the 1500’s. [See sources below] The custom of fasting on Erev Rosh Chodesh is first brought in the early Acharonim ibid [Manos Halevi and onwards] as the custom of Chassidim and Anshei Maaseh. This custom is not recorded in Rishonim or in the Michaber and Rama. See Avos Hachassidus of the Rebbe Rayatz [printed in Pardes Chabad 8] chapter 32-33 for a vivid description of the experience of Yom Kippur Katan in the 1600’s
 Peri Chadash 417 in name of Ramak; Siddur Yaavetz; M”B 417/3; See Rameh 79 and Shelah 120b; Or Hatorah Bo p. 253
 The reason: The reason the Ramak called the day of Erev Rosh Chodesh as Yom Kippur Katan is because on this day one receives atonement for all the sins of the previous month. This is similar to the goat offering on Rosh Chodesh which would bring atonement for sins. [Peri Chadash in name of Ramak ibid]
 M”A 417/3; Manos Halevi [of Rav Shlomo Alkabetz]; Admur 249/12-13 and Kuntres Acharon 249/5 “The fast of Erev Rosh Chodesh that has become widespread in the later generations”; Kneses Hagedola 417 in name of Manos Halevi; Bach 686; Elya Raba 518; Peri Chadash 417; Birkeiy Yosef 417/1; Siddur Yaavetz; Reishis Chochmah Shaar Hateshuvah 4/21; Kaf Hachaim 417/10-26; M”B 417/4; Hashlama of Divrei Nechemia 670; See Avos Hachassidus of the Rebbe Rayatz [printed in Pardes Chabad 8].
Background: The custom of fasting on Erev Rosh Chodesh is first brought in the early Acharonim ibid [Manos Halevi and onwards] as the custom of Chassidim and Anshei Maaseh. This custom is not recorded in Rishonim or in the Michaber and Rama.
The reason: The reason behind the custom to fast is because on Rosh Chodesh they used to bring the Karban Sair, the goat offering, which was atonement for Hashem for Him diminishing the moon. Hence, pious Jews would fast as a replacement of this Karban and atonement. [Kneses Hagedola ibid in name of Manos Halevi, brought in Kaf Hachaim 417/10] Alternatively, the reason for this fast is because it states in Sefer Chassidim 97 that one should not pass 30 days without fasting. Likewise, the Acharonim record that fasting once every thirty days is good for one’s health. [Kaf Hachaim 417/10]
Fasting from the day of the Molad: Some would fast from the day of the Molad until Rosh Chodesh. [Mekubal brought in M”A ibid]
Accepting the fast from the time of Mincha: Those who are accustomed to fast on Erev Rosh Chodesh are to accept the fast during Mincha of the day before Erev Rosh Chodesh. [Biur Halacha 562 “Sheragil”]
Bli Neder: Those that desire to take upon themselves the custom to fast are to make a stipulation that they are not doing so forever, in order to avoid it becoming a Neder. [Radbaz 2/277; M”B 417/4]
Rosh Chodesh falls on Sunday: In the event that Rosh Chodesh falls on Sunday, the fast takes place on Thursday. [M”A 417/3]
Rosh Chodesh falls on Shabbos: In the event that Rosh Chodesh falls on Shabbos, the fast takes place on Friday. [Admur 249/12] See 249/12-13 regarding if one must fast until nightfall. See our Sefer “The Laws of Erev Shabbos” Chapter 5 Halacha 2. This is dependent on whether he ever fasted on Erev Rosh Chodesh until nightfall, and on whether he accepted the fast during the Mincha before.
Erev Rosh Chodesh Teves and Iyar: One does not fast on Erev Rosh Chodesh Iyar. [Admur 429/9; M”A 429/5] The same applies to Erev Rosh Chodesh Teves.  One likewise does not fast on Erev Chanukah to make up for this fast. [Hashlama of Divrei Nechemia 670 “It is not such a stringent fast”; See Tashis Lerosho p. 368]
 Admur Kuntres Acharon 249/5
 M”A 417/3; Rameh 79; Peri Chadash 417; M”B 417/3
 Peri Chadash 417; M”B 417/3; Kaf Hachaim 417/14
 Shesilei Zeisim 417/3
 Peri Chadash 417; Kaf Hachaim ibid
 Yeshuos Chochmah 97/1; See Yerushalmi Sanhedrin 10/2; Igeres Hateshuvah 3
 See Avos Hachassidus of the Rebbe Rayatz [printed in Pardes Chabad 8] chapter 32-33 that even in the 1600’s the marking of Yom Kippur Katan in its proper method began to decrease from previous generations. See also Kuntres Umayan p. 25 for a Sicha of the Rebbe Rayatz that describes how in the city of Vilna the Rabbanim began following leniencies in Yom Kippur Katan and this aroused the anger of Rav Chaim Ozer, the Rav of Vilna. When one of these Rabbanim ridiculed the Chassidim for making a Holiday of Yud Tes Kislev, Rav Chaim Ozer responded “By them they are adding in serving G-d, by us we are subtracting”; See Igeres Hateshuvah chapter 3
 Otzer Minhagei Chabad Elul p. 5; See Sefer Hasichos 1989 Shemos
 See Igeres Hateshuvah chapter 3; Likkutei Sichos 23/242; Siddur Yaavetz
 M”A 417/3 “The Selichos is to be said before Mincha”; Peri Chadash 417 brings the Seder of prayers that were accustomed to be recited in Jerusalem on Erev Rosh Chodesh. M”B 417/3 “Some are accustomed to perform Seder Yom Kippur Katan by Mincha” See Avos Hachassidus of the Rebbe Rayatz [printed in Pardes Chabad 8] chapter 32-33 for a vivid description of the Seder Yom Kippur Katan in the 1600’s
 The custom of saying Selichos on Erev Rosh Chodesh is seemingly an offshoot of the custom to fast, as Selichos is customarily recited on a fast day. [See 565/5] Nevertheless, see the Poskim in next footnote who do not write that the saying of the Selichos was contingent on fasting that day, and so is the custom today amongst many, that although they are not accustomed to fast they recite the Seder of Yom Kippur Katan by Mincha.
 Otzer Minhagei Chabad Elul p.5
 M”A 417/3
The reason: As after Mincha it’s already considered like Rosh Chodesh. [M”A ibid]
 M”B 417/4
 Mateh Efraim 581/3; Kitzur SHU”A 127/1; Kaf Hachaim 581/23
 See Shaar Yissachar Elul that the second 40 days which achieved the appeasement of G-d after the sin of the Eigel concluded on the 29th of Menachem Av. It is thus called Yom Kippur Katan as it is similar to the third set of 40 days which concluded on Yom Kippur. It is due to this month that all the other Erev Rosh Chodesh are also called Yom Kippur Katan.
 Otzer Minhagei Chabad 4
 Moreh Baetzba 147; Kaf Hachaim 417/11; M”B 417/3
The reason: As this is a day of atonement similar to Erev Rosh Hashanah which is the last day of the year. [M”B ibid; Kaf Hachaim 417/26]
 M”B ibid; Kaf Hachaim 417/26
 Moreh Baetzba ibid
 Pokeiach Ivrim Chapter 1/5 and 3
 Rama 420/2 regarding Erev Shabbos after midday, and the same would apply to Erev Rosh Chodesh being Tachanun is omitted starting from midday; P”M 420 A”A 1 that so is custom of Prague and other cities; Maareh Kohen Y.D. 401/2
 The reason: As this comes to lead one to eulogize. [M”B 420/3]
Other opinions: Some Poskim rule that Tziduk Hadin and Kaddish is recited on Erev Rosh Chodesh and even on Rosh Chodesh being it is not considered a eulogy but merely a blessing to Hashem. [Michaber 420/2; Opinions in Tur 420; Talmidei Rashi; Rambam] Practically, each community is to follow their custom, and in a place where there is no set custom, it is better not to say it. [Kaf Hachaim 420/1]
 Shach Y.D. 401/2; Beir Heiytiv 401/1; Nitei Gavriel Aveilus 52/15
 Admur 292/7; Siddur Admur; Hashlama of Rebbe Nechemia 131/8
 Admur 292/7; Siddur Admur; M”A 292/3-4; Darkei Moshe 292; Tur 292
 Michaber 425/2
 Rama 425/1; Sefer Haminhagim p. 64 [English]
 Rama ibid; Sefer Haminhagim ibid
 Sefer Haminhagim ibid; See Chapter 3 Halacha 11C!
 See 425/1; Sefer Haminhagim p. 64 [English]
 Rama 425/2; Sefer Haminhagim p. 64 [English]
 Sefer Haminhagim p. 64 [English]; Likkutei Sichos 35/27; Igros Kodesh 5/108 [letter to Rav A.C. Naah printed in Shulchan Menachem p. 96-105]
 Sefer Haminhagim p. 49 [English]; Hayom Yom 30th Sivan
The ruling of Admur in the Shulchan Aruch and other opinions: In a case that there are two Haftoras that week one is only to read the Haftorah which will be read in public and not the Haftorah of the weekly Parsha. The simple reason behind this ruling is as Admur explained earlier that the entire custom to read the weekly Haftorah is only in order so one be prepared in case he is called up to read it. Hence in a week that it is not being read there is no custom to review it. [Admur ibid; M”A 285/12] Nevertheless our custom is to review all the applicable Haftoras. Some Poskim rule one is to only read the weekly Haftorah and not the additional Haftorah. [Kneses Hagedola 285; Moreh Betzba4/132; Ben Ish Chaiy Lech Lecha 11; Kaf Hachaim 285/36 that so is custom.]
 Kaf Hachaim [Falagi] 32/17
 Sichos Kodesh 1976 p. 663; Likkutei Sichos 15 p. 549; Sefer Haminhagim p. 77
 Kaf Hachaim 581/98; Alef Hamagen 581/110; Siddur Marshkov; Custom of Rebbe