No Tahanunim the Whole Nissan - Why?
Halacha Yomit answers that it has to do with the past, present, and future.
Past - the Nesi'im would bring their Korbanot on days 1-12. Day 13 was their Isru Hag.
Present - the 14th is Erev Pesah. Days 15-21 are Pesah. Day 22 is Isru Hag of Pesah.
Future - The third Beit Hamikdash will be built on the first day of Pesah (via Heaven), and the dedication of the third Beit Hamikdash will last 7 days (days 23-29) since one may not hold the dedication of the Beit Hamikdash on Pesah because of the Halachic dictum of "אין מערבין שמחה בשמחה" (we do not combine one joyous occasion with another).
(And, of course, day 30 is Rosh Hodesh.)
So, it comes out that the entire month is dedicated for days of joy for Israel. It is therefore not a month for supplications or fasting.
Wouldn't it be nice if the third Beit Hamikdash were built this Pesah?