Siman 107ק״ז
1 א

כָּל חֹדֶשׁ נִיסָן אֵין אוֹמְרִים תַּחֲנוּן, וְלֹא צִדּוּק הַדִּין, וְאֵין אוֹמְרִים צִדְקָתְךָ בַּשַׁבָּת בְּמִנְחָה. נוֹהֲגִין מֵרֹאשׁ חֹדֶשׁ וָאֵילֵךְ לִקְרוֹת בְּכָל יוֹם פָּרָשַׁת הַנָּשִׂיא שֶׁהִקְרִיב בּוֹ בַּיּוֹם, וּבְיוֹם שְׁלשָׁה עָשָׂר קוֹרִין פָּרָשַׁת בְּהַעֲלֹתְךָ, עַד כֵּן עָשָׂה אֶת הַמְּנוֹרָה, שֶׁהִיא כְּנֶגֶד שֵׁבֶט לֵוִי.

During the entire month of Nisan we do not say Tachanun1See Chapter 22. or Tzidduk Hadin2See Chapter 198:14. (burial service), and we do not say Tzidkasecha on Shabbos at Minchah.3Shulchan Aruch 429. It is customary4This custom is not very widespread in our times, except among Chassidim. from Rosh Chodesh on, to read (from the Seifer Torah) the parashah of the Nasi5The leader of each tribe was called Nasi. On Rosh Chodesh Nisan the Tabernacle was erected, and in honor of its dedication, during the ensuing twelve days, each Nasi had his day on which he brought a special offering, as is written in Numbers 7:10–7:89. who brought his offering on that day. On the thirteenth day of the month, we read in parashas Beha'alosechah until "Kein asah es hamenorah" "So did he make the Menorah" which is fitting for the tribe of Levi.6Aaron, the High Priest, and Nasi of the tribe of Levi did not take part in the offering of the Nesi’im. His lighting of the Menorah was his part in the dedication. See Rashi. (Numbers 8:2)

2 ב

אֵין מִתְעַנִּין בּוֹ אֲפִלּוּ תַּעֲנִית יַארְצַיְיט (יוֹם זִכָּרּוֹן לְהוֹרָיו). אֲבָל תַּעֲנִית חֲלוֹם, מִתְעַנִּין. וְהַבְּכוֹרִים, מִתְעַנִּין בְּעֶרֶב פֶּסַח, כַּאֲשֶׁר יְבֹאַר אִם יִרְצֶה הַשֵּׁם בְּסִימָן קי"ג. וְחָתָן וְכַלָּה גַּם כֵּן מִתְעַנִּין בּוֹ, וַאֲפִלּוּ בְּרֹאשׁ חֹדֶשׁ נִיסָן (תכט).

[During the month of Nisan,] we do not fast even the fast of a Yahrzeit,7The anniversary of the death of one’s father or mother. (See Ramah 429:2) This is our custom; but Shulchan Aruch permits a private individual to fast during Nisan. but we may fast because of a dream.8If one has a particularly onimous and disturbing dream, fasting with repentance helps to nullify the decree indicated by the dream. The firstborn (males) fast on erev Pesach, as will be explained, God willing, in Chapter 113. A groom and a bride also fast during Nisan,9Since it is a day of atonement for them, they fast the day of the wedding until the marriage ceremony takes place. (Magein Avraham 573) Refraining from fasting during Nisan is not a halachah, but a custom, and this fast overrides it. (poskim) even on Rosh Chodesh Nisan.10Ramah 573. This is so only on Rosh Chodesh Nisan, because two of Aaron’s sons (Nadav and Avihu) died on that day. On any other Rosh Chodesh, the bride and groom may not fast. (Magein Avraham, Mishnah Berurah 573:9)

3 ג

בַּשַׁבָּת הַגָּדוֹל לְמִנְחָה, נוֹהֲגִין שֶׁאֵין אוֹמְרִים בָּרְכִי נַפְשִׁי, אֶלָּא עֲבָדִים הָיִינוּ וְכוּ'. לְפִי שֶׁבַּשַׁבָּת הַגָּדוֹל הָיְתָה הַתְחָלַת הַגְּאֻלָּה וְהַנִּסִּים (סִימָן ת"ל).

On Shabbos Hagadol11The Shabbos before Pesach is so called because in Egypt, the Jews were commanded to take a lamb on the tenth day of Nisan and designate it for the paschal offering. They did so, and tied the sheep to their bedposts. The Egyptians, upon hearing the Jews were bringing the sheep as a sacrifice, did not protest, even though the lamb was the Egyptian idol. That year, the tenth day of Nisan occurred on Shabbos, and in commemoration of this miracle, we call it Shabbos Hagadol—the Great Shabbos. (Tur 430 etc.) at Minchah it is customary not to recite Barchi Nafshi, but to say instead Avadim Hayinu12This is found in the Hagaddah of Pesach. It is customary to recite until Lechappeir al Kol Avonoseinu (to atone all our sins). (Ramah 430) The Vilna Gaon did not agree with this custom, and many people follow his ruling and do not say it. etc., because on Shabbos Hagadol the redemption from Egypt and the miracles began.13See note 11.