Rosh Chodesh Nissan:
A. Omitting Tachanun and other prayers of supplication in the month of Nisan:
The custom in these provinces is that throughout the entire month of Nissan the following prayers are omitted:
- Tachanun: Tachanun is omitted daily. Vehu Rachum [added in Tachanun on Mondays and Thursdays] is omitted on Mondays and Thursdays.
- Kel Erech Apayim: Kel Erech Apayim is omitted prior to the Torah reading on Mondays and Thursdays.
- Yehi Ratzon: The Yehi Ratzon customarily said after Kerias Hatorah is omitted. [It is not the Chabad custom to recite Yehi Ratzon after the Torah reading even on other days of the year.]
- Lamnatzeiach: Is omitted daily prior to Uva Letzion, [however it is recited after Davening].
- Tefila Ledavid: The Psalm of Tefila Ledavid is omitted on all days that Tachanun is not recited.
- Yizkor: Yizkor is not recited throughout the month of Nissan, with exception to the last day opf Pesach.
- Av Harachamim: Is omitted on Shabbos before Musaf, [with exception to the last day of Pesach, due to Yizkar, for which it is recited].
- Tzidkascha Tzedek: Tzidkascha Tzedek is omitted after Mincha Shemoneh Esrei of Shabbos.
- Tziduk Hadin: One does not say Tziduk Hadin and the Kaddish that follwos it throughout the month of Nissan. This applies even the person who died is a great Torah scholar and is the Gadol Hador.
- Eulogies: Eulogies are not given throughout the month of Nissan, with exception to a Torah Scholar, before he is buried. This applies even if the Torah scholar is not of the greatest caliber.
Is one to increase in Simcha in the month of Nissan?
Rashi [on Taanis 29a] explains the reason for the increasing in joy in the month of Adar is due to the miracles that occurred on Purim and Pesach. Some Poskim rule based on this that one is to increase in joy also in the month of Nissan. Others however explain that Rashi does not intend to novelize a new law of increasing in joy in Nissan but rather is explaining why the month of Adar is meritorious.
Uniqueness of Nissan:
This month is one of a miraculous nature. It is an auspicious time for one to be able to accomplish spiritually that which to him seems a distance away.
Every day of the month is like Rosh Chodesh:
Every day of the month of Nissan is like Rosh Chodesh. Just like Rosh Chodesh is not a day of work and is a slight holiday, so too the entire month of Nissan.
Some are accustomed not to visit cemeteries throughout the month of Nissan. If one has a Yartzite in Nissan he is to visit the grave on Erev Rosh Chodesh Nissan. There are those who rule that visiting the gravesites of Tzaddikim is an exception and may be visited anytime throughout the month. Many however are accustomed to visit cemeteries during the month of Nissan just like any other time of the year. The Rebbe was accustomed to visit the Tziyon of the Rebbe Rayatz in the month of Nissan on various occasions.
One does not fast in the month of Nissan, even a Taanis Yachid, and certainly not a Taanis Tzibur.
A Chasan and Kallah: A Chasan and Kallah that are getting married during Nissan, are accustomed to fast on the day of their wedding. Even on Rosh Chodesh Nisson they are required to fast if their wedding is taking place that day [or that night after sunset]. However on Isru Chag, a Chasan and Kallah do not fast [as will be explained below].
Taanis Chalom: One may fast a Taanis Chalom during Nissan. One is not required to make up the fast the next month. It is permitted to fast on Sunday during Nissan to atone for a Taanis Chalom that was performed the previous Shabbos. This applies even if that Sunday is Rosh Chodesh Nissan.
Taanis Yartzite: A son does not fast on the day of the Yartzite of his father or mother throughout the month of Nissan.
Yom Kippur Katan: One does not fast on the last day of Nissan, which is Erev Rosh Chodesh Iyar [known as Yom Kippur Katan, of which there are people that are accustomed to fast].
D. The reason for omitting Tachanun and not fasting:
All the above is not prohibited to be done according to the letter of the law, but is rather a mere custom that was introduced in the later generations. It is based on the teaching of our sages, that starting from the first of Nissan the Nessi’im began bringing sacrifices for the inauguration of the altar. Each Nassi brought a Karban on his designated day until [but not including] the 13th of Nissan. The day that each Nassi brought his sacrifice was considered to be his Holiday [and thus up until the 13th of Nissan it was considered like a Holiday]. On the 14th of Nissan all of Israel would bring their Pesach sacrifice, and thus it was a holiday for all Israel. [For this reason we are accustomed to omit Tachanun on these days]. [Now, with regards to the remaining days] since afterwards comes 8 days of Pesach, which are a Holiday, therefore it ends up that majority of the month is holy, as only the 13th, [and past the 8th day of Pesach] are non-holidays. Thus, the custom is to treat the entire month with holiness similar to a holiday and omit Tachanun and avoid fasting.
E. The reading of the Nassi:
Starting from the first of Nissan until the 13th of Nissan it is customary to read the paragraph of that day’s Nassi.
What to read on the 1st/last day: On the first day of Nissan one begins to read from “Veyihi Biyom Kalos Moshe“. On the 13th day one begins to read from “Zos Chanukas Hamizbeiach”.
Saying the Yehi Ratzon: The Yehi Ratzon prayer is recited after the reading of each day’s Nassi. It is recited even by a Levite or Kohen.
Reading the Nessim from a Sefer Torah: It is not the Chabad custom to read the portion of the Nessim from the Torah scroll. However some are accustomed to do so, and those that do so have upon whom to rely. However those who follow this custom should not read the portion in the Torah after the Torah reading on Shabbos, but rather after Musaf.
When should the Nassi be read? The Rebbe Rayatz writes to read it after Davening, before Tehillim, although the custom of the Rebbe was to read it after Tehillim.
 429/8; Michaber 429/2; Rokeaich 245 based on Miseches Sofrim 21/1-3
 Siddur Tehillas Hashem [from 1978 and onwards with the Rebbe’s approval] based on Sichas Kodesh 6th Tishrei 1975 that the Rebbe equates Kel Erech Apayim with Lamnatzeiach. See Shulchan Menachem 3/293; Glosses of Rav Raskin p. 165; Hiskashrus 438 p. 17; To note however that the Rebbe himself did recite Kel Erech Apayim even when Tachanun was not recited. [Rav Raskin ibid]
Ruling of Admur in the Shulchan Aruch: In the Shulchan Aruch [429/12; 602/4] Admur rules that Kel Erech Apayim is recited with exception to Erev Pesach. This follows the ruling of the Hagahos Maimanis. [Some explain that it is for this reason that Admur in 602/4 placed this ruling in parentheses, as it is not brought in previous Poskim in Shulchan Aruch. [Shulchan Menachem ibid footnote 2] Vetzaruch Iyun as it is brought by Admur in 429/12 without parentheses, and is likewise clearly implied from the Rama 429/2] To note that also in the Siddur before Kel Erech Apayim Admur lists a number of days that it is to be omitted on, thus implying it is not always omitted when Tachanun is not said.
Ruling and explanation of Divrei Nechemia: The Divrei Nechemia 131/9 brings three opinions regarding the saying of Keil Erech Apayim: 1) It is only omitted by a day that Halel is recited or a day that has the status of a Yom Tov like Erev Pesach. [This is the opinion of the Rama 429/2] 2) It is omitted on every Erev Yom Tov and on Isru Chag. 3) Every day that Tachanun is omitted so is Lamnatzeiach. [so rules Peri Chadash 131] The Divrei Nechemia concludes that regarding Lamnatzeiach we rule like the third opinion, however regarding Keil Erech Apayim we rule like the opinion that rules it is to be recited.
The reason behind the Rebbe’s ruling: The Rebbe explains that the ruling in the Shulchan Aruch of Admur follows the ruling of the Rama that differentiates between the laws of Tachanun and that of Lamnatzeiach. However according to Admur in the Siddur that rules Lamnatzeiach is always omitted when Tachanun is not said then likewise Kel Erech Apayim is to be omitted in all days that Tachanun is not said. [Sichas Kodesh ibid, printed in Shulchan Menachem ibid] As for the reason why Kel Erech Apayim is omitted this is because it mentions sin, and we do not desire to mention sin on any day that Tachanun is omitted. [See Shaar Hakolel 36/2; Shulchan Menachem ibid footnote 5]
 Recited normally on Mondays and Thursdays prior to the opening of the Aron.
 Recited in many communities after the Torah reading. It is not the Chabad custom to recite these prayers anytime during the year.
 Admur in Siddur based on Kneses Hagedola 131/3; Peri Chadash 131/1; Mamar Mordechai 131/6 that whenever Tachanun is omitted Lamnatzeiach is omitted; Divrei Nechemia 131/9; Kaf Hachaim 581/78 states that so is the custom of Sefaradim.
Ruling of Admur in Shulchan Aruch and other Poskim: Admur rules in 429/12 that Lamnatzeaich is recited during the month of Nissan with exception to Erev Pesach. This follows the ruling of the following Poskim: Rama 131/1 and 429/2; Maharil Nissan; Minhagim Tirna p. 167
 Sefer Hammarim 1948 p. 268; Sefer Hmainhagim p. 19; This Takana began on the 2nd of Nissan 1944 based on what the Rebbe Rayatz heard in Gan Eden
 Siddur Admur
 The reason: The reason for this is because this Psalm represents supplications and is hence omitted by times of joy. [Shaar Hakolel 11/23 based on Seder Hayom]
 Admur ibid; Chok Yaakov 429/7
 Admur 429/8 in parentheses
 Implication of Admur ibid that this statement is also going on Av Harachamim
 Admur ibid; Rama ibid
 Admur ibid; Michaber ibid; Rokeiach ibid
 Elya Raba 685/8; Chasam Sofer 160 based on Rashi Taanis 29a
 Shieilas Yaavetz 2/88; Likkutei Sichos 16 p. 344
The Rebbe [Likkutei Sichos 16/345] explains Rashi to mean that the entire month is a meritourious month for the Jewish people due to the fact that Moshe Rabbeinu was born on this month, and it is due to his merit that we merited the miracles of Purim and Pesach. This is why Rashi also mentioned Pesach in his explanation. Shieilas Yaavetz 2/88 explains the reason for mentioning Pesach is because this shows that the miracles continued for two months and hence we increase in joy in Adar and not Chanukah, as by Chanukah the miracles were for only one month.
 Shlah Hakadosh end of Parshas Bo
 Nitei Gavriel Pesach 3/8 [p. 54]
 See Nitei Gavriel ibid; Gesher Hachaim 1 p. 259
 Gesher Hachaim 1 p. 259
 Otzer Minhagei Chabad 15 [p. 13]
 429/9; Michaber 429/2; Miseches Sofrim 21/3
 Taanis Bechoros: Tzaruch Iyun why Taanis Bechoros is allowed during Nissan if the custom is not to fast and not to set any public fasts. Perhaps Taanis Bechoros is different as it is the only available day to fast for this occasion. Veztaruch Iyun!
 Meaning that even if they are getting married at night when it is already the second day of Nissan, nevertheless they are to fast on Rosh Chodesh as that is considered the day prior to their wedding.
 429/9; Rama 573/1
The reason: It is permitted for a Chasan to fast on Rosh Chodesh Nissan being that some say one is to always fast on this date, as on Rosh Chodesh Nissan Nadav and Avihu passed away. [Rama ibid; Michaber 580/1-2; Admur 288/3]
 429/10; 288/3
 The reason: As even on Shabbos and Yom Tov one is allowed to fast a Taanis Chalom. [ibid]
 The reason: Being that the prohibition of fasting on the weekdays of this month is only a custom, [and not the letter of the law, as will be explained] therefore, one who fasts due to a dream during the month, which is allowed even on Shabbos and Yom Tov, does not need to fast in Iyar to atone for the sin that he did by fasting in Nissan, as is needed to be done when one fasts on Shabbos. [Admur ibid]
 429/11, 288/3. Regarding Isru Chag-see below
 Admur 429/9; Rama 429/2; Admur 429/17 regarding Issru Chag
 Admur 429/9; M”A 429/5
 429/9; M”A 429/3; Peri Chadash 429/2; Olas Shabbos 429/2; Miseches Sofrim 21/3
 Meaning the omission of Tachanun and the ruling that one is not to fast.
 Customarily the day that each person brought a sacrifice was considered a holiday for him.
 429/15; Shlah Pesachim 140; Chok Yaakov 429/10
 After the inauguration of the Tabernacle, each day a different Nassi of a tribe offered a sacrifice to the Temple altar. Thus in commemoration of this we read the Nassi each day in correspondence to the 12 offerings of the Nassim. The sacrifices are recorded in Parshas Naso, and have been reprinted in the Tehilas Hashem Siddurim.
 Shaar Hakolel in Siddur Torah Oar; This follows the opinion of the Rama in 684/1 regarding the Chanukah reading
Other Opinions: Some hold that one should begin from Birchas Kohanim. [See Otzer Minhagei Chabad and Nitei Gavriel]
 Shaar Hakolel in siddur Torah Oar
Ruling of Admur in Shulchan Aruch and Other Opinions: In 429/15 Admur rules to read from Behaaloscha until Kein Asah Es Haminorah, being that this paragraph corresponds to the tribe of Levi. In Kuntrus Hasiddur of Grach Nah he rules like Admur here. The Rebbe however ruled to follow the opinion of the Shaar Hakolel printed in Torah Oar. [Otzer Minhagei Chabad Nissan page 7]
 Shaareiy Halacha Uminhag 2/189: The Rebbe publicized that the Yehi Ratzon prayer is to be said after the reading.
Other Opinions: The saying of Yehi Ratzon was not recorded by Admur neither in his Shulchan Aruch nor his Siddur. The Rebbe Rashab was not accustomed to say this Yehi Ratzon and actually directed others not to say it. There are many [non-Chabad Chassidim] which are accustomed not to say it. [See Otzer Mihagei Chabad page 4-6]
 Shaareiy Halacha Uminhag 2/189 [page 80]; Hayom Yom 1st of Nissan based on directive of Rebbe Rashab
The reason: As the soul of every Jew incorporates the souls of all other Jews and all other tribes, and thus receives divine arousal from each days reading. Nevertheless one who reads the paragraph of his tribe receives this revelation in a much more revealed and powerful form. [Rebbe Shaar Halacha Uminhag 2/189]
 Shaareiy Halacha Uminhag 2 pages 77-79
 As was seen by the Friedeker Rebbe, and above by his father. Similarly they were not stringent to read Snayim Mikra from a sefer Torah [Rebbe ibid]
 See Nitei Gavriel Pesach 1
 see Chazon Ovadia Pesach which explains in length that there is no prohibition involved here of “moving a sefer Torah for no reason”
 Otzer Minhagei Chabad Nissan page 3