Why did he mourn?
Rashi (1): He saw that Koresh ceased building the Beis ha'Mikdash. He commanded to begin, and retracted due to what Rechum Be'el Te'em and Shimshai Safra, haters of Yehudah and Binyamin, wrote (Ezra 4:8).
How much is "Sheloshah Shavu'im Yamim"?
Rashi: It is 21 years. There are 18 years from the first year of Daryavesh ha'Midi, that he put to his heart to calculate the 70 years of Galus, until the second year of Daryavesh ha'Parsi ben Esther. I do not know whether he began fasting three years beforehand, or if he continued fasts that he accepted three years after building [started again].
Ibn Ezra: It was three weeks, in Nisan 1 . Malbim citing Mahari ? he saw three kingdoms that will rule over Yisrael 2 , and fasted one week for each.
Ibn Ezra: How could he mourn during Pesach, and not drink wine on Seder night? It says "v'Sar Malchus Paras Omed l'Negdi Esrim v'Echad Yom" ? the three weeks that he fasted. This was on the 24th of the first month ? however, this could be Nisan, or the first month of Koresh's third year.
Bavel is not included, for its rule over Yisrael already ceased. (PF)
