סימן קז סעיף 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)
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)