Tach’nun is not said the entire month of Nissan, nor is צדקתך said on Shabbos at Minchah during this month. Some people follow the custom that every day from ראש חודש ניסן until the thirteenth of Nissan they read after Shacharis that day’s chapter of the Service of the Consecration of the Mishkan, for it was in the month of Nissan, in the year following יציאת מצרים that the Mishkan was erected. (Some congregations have the custom to have this reading from a Sefer Torah, although nobody is called up to this reading.) The custom generally prevails that at Minchah of שבת הגדול, “the Great Shabbos,” so called because it marked the beginning of our actual deliverance from Egypt, instead of "שיר הכבוד" we say part of the Haggadah, namely, from "עבדים היינו" until "לכפר על כל עוונותינו".
