The month of Nissan is a joyous time for Israel; it has been in the past, it is in the present, and will continue to be in the future. This is because on the first of Nissan the Mishkan (Tabernacle) was erected and the twelve leaders of the twelve tribes of Israel commenced bringing their sacrifices for the inauguration of the Mizbe’ach (altar). The day following the twelfth day, meaning the thirteenth of Nissan, was Isru Chag for them and the fourteenth of Nissan was already Erev Pesach.
After this, the seven days of Pesach commenced and the twenty-second of Nissan was Isru Chag. The building of the third Bet HaMikdash, which we pray should be speedily in our days, will take place on the first day of Pesach, and this is why this day will be especially joyous forever.
Although one may not build the Bet HaMikdash on Yom Tov, for this building entails several works that are prohibited on Yom Tov, nevertheless, the third Bet HaMikdash shall be built by Hashem, and this is indeed permissible on Yom Tov. As it is written, “In Nissan they were redeemed and in Nissan they shall once again be redeemed in the future”, as the verse states, “As it was in the days you left Egypt, I shall show them wonders.” The inauguration of the third Bet HaMikdash shall last for seven days, but will not be able to be celebrated until after the holiday of Pesach, for two joyous occasions may not be combined (it is for this reason that one may not get married during the holiday). It comes out that the entire month of Nissan is dedicated to joy and happiness for Israel. Thus, it is improper to recite Viduy and Tachanun (supplication prayers) before Hashem during these days and we therefore do not recite Nefilat Apayim (Tachanun prayer) during the entire month of Nissan.
Similarly, a fast cannot be decreed upon the public during this month. Nevertheless, one would customarily fast in honor of his either of his parents’ anniversary of death during this month. (This means that it is customary to fast on the anniversary of one’s parent’s demise and this fast should not be pushed off, even if it falls out in the month of Nissan.)
