107:1 The entire month of Nisan, we do not recite Tachanun, nor Tziduk HaDin (which is a prayer recited at funerals). Also, we do not recite Tzidkascha Tzedek at the Shabbos mincha service (nor do we recite the “yehi ratzons” that follow Torah reading on Mondays and Thursdays – Mishnah Brurah 429:8). The practice from Rosh Chodesh Nisan on is to recite each day the Torah section about the leader of each Tribe who brought his offering on that day of the month (see Numbers chapter 7). On the thirteenth of Nisan, we read parshas Behaaloscha until “thus he made the menorah” (Numbers 8:4), which corresponds to the Tribe of Levi.
107:2 We do not fast the entire month, not even on a yahrzeit (or on erev Rosh Chodesh Iyar, even for those who normally fast erev Rosh Chodesh – MB 429:10), but one may fast for a bad dream. The first-born sons do fast on the eve of Pesach, as will be explained in chapter 113. A bride and groom do fast on it, even if their wedding day is Rosh Chodesh Nisan. (They would not normally fast on Rosh Chodesh but Rosh Chodesh Nisan is different because Nadav and Avihu, the sons of Aaron, died on that day – Mishnah Brurah 573:9.)