Talmud su Esodo 40:2

And why do we not fast in the month of Nisan? Because on the first of Nisan the Tabernacle was erected⁴Cf. Ex. 40, 2. and twelve princes offered their sacrifices during [the first] twelve days, a day for each tribe,⁵Cf. Num. 7, 1ff. and everyone held a festival on his day.⁶Between the twelfth of Nisan and the fifteenth, on which Passover begins, only two days intervene; therefore they, too, are regarded as festival days. The Passover ending on the twenty-first (in the land of Israel) or on the twenty-second (in the Diaspora), only eight or nine days of the month remain. As by far the greater part of the month was made up of festival days, the entire month is treated as a festival season in which fasting is prohibited. Furthermore, in the time to come the Sanctuary will be rebuilt in Nisan, to confirm what is stated, There is nothing new under the sun.⁷Eccl. 1, 9.