1. The prayer of Tachanunis not recited during the entire month of Nissan. Similarly, fasting is not allowed during the entire month. [According to the Shulchan Aruch this refers to not announcing a fast in public; according to the Rama this refers even to a private fast, including not fasting on a yartzeit for one’s parents.] The only exceptions to this is fasting after experiencing a disturbing dream and the “fast of the firstborn” on erev Pesach (Shulchan Aruch and Mishnah Berurah 429).
2. The reason for this is that since the Tabernacle in the Wilderness was inaugurated on the first of Nissan and from that day on, for the next twelve days, the Princes (Nesi’im) brought their inauguration offerings – one prince per day – therefore each day of bringing an offering is like a festival. In addition, erev Pesachand the seven days of Pesach itself is another eight days of holiness. And since, therefore, the majority of the month were days of holiness, we treat the entire month as holy. Some say that in the Future the Holy Temple will be built on the festival of Pesach and the inauguration of its Altar will be held during the seven days following Pesach. We therefore treat those days as holy, in line with the future (ibid. and Kaf HaChaim ad loc.).
