סימן קז סעיף 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)
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.)