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 offering their sacrifices in honor of 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 followed by the Twenty-Second of Nissan which was Isru Chag. The building of the third Bet Hamikdash, which we pray should take place speedily and in our days, will take place on the first day of Pesach and this is why this day will be especially joyous forever.
Although the Bet Hamikdash may not be built on Yom Tov, for building entails several works that are prohibited on Yom Tov, nevertheless, the third Bet Hamikdash shall be built by Hashem which is indeed permissible on Yom Tov. 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 begin 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). Thus, the entire month of Nissan is dedicated to joy and happiness for the Jewish nation. It is therefore improper to recite Viduy and Tachanun (supplication prayers) during these days and it is for this reason that we do not reciteNefilat Apayim (Tachanun prayer) during the entire month of Nissan.
Similarly, a fast cannot be decreed upon the public during this month. Nevertheless, one should customarily fast in honor of the anniversary of the passing of either of one’s parents during this month. (This means that it is customary to fast on the anniversary of a parent’s death and this fast should not be pushed off, even if it falls out during the month of Nissan.)