We mentioned that the final days of the month of Nissan are considered to be auspicious days because the majority of the month was spent in Qedusha (holiness).
(See
http://www.atorahminute.com/2010-03-14).
Additionally, there is an opinion that the final days of Nissan contain their own sanctity.
According to the Ma'aseh Roqeyah, in the future, the third Beth Hammiqdash will be built, not during the Yom Tob (holy days of Passover), but after the holiday of Pesah, because if it were built during the holiday itself, it would be considered mixing one rejoicing with another. Therefore, the dedication of the Temple, which would take another seven days, would bring us to the end of the month of Nissan.
That is why even the final days of the month are also considered holy, and fasting is not permitted, in recognition of the third Beth Hammiqdash (Temple) which will be built in the month of Nissan. May we witness it speedily in our days, Amen.
(See Kaf Hahayim 429, Oth, 21)
