As pointed out here, and noted in the comments, there is no Taḥanun recited the entire month of Nisan. According to MB 429:2(7) it's because of Kedushah.
"What is the defining difference between Kedushah and Simchah in terms of saying Tachanun? We certainly recite it in Eretz Yisrael and in Yerushalayim, where there is a higher level of Kedushah, and we do not recite it when there is a groom present, so is there something about a day - or month - of Kedushah that gives it some level of Simchah that eliminates the need for reciting Tachanun, or does a certain level of Simchah create an atmosphere of Kedushah (or vice versa)? Or does there need to be some level of Kedushah in a Simchah-dik atmosphere together?"