Commentary for Daniel 10:2

had been mourning when he saw that Cyrus had curtailed the construction of the Temple, for he had ordered to commence it, and he reneged because of the missive of the adversaries of Judah and Benjamin: Rehum the adviser and Shimshai the scribe, as is written in the Book of Ezra (4:5).

three weeks of days twenty-one years. They are eighteen years from the one year of Darius the Mede, when he [Daniel] put to his heart to calculate the seventy years of the exile, as it is written: until the second year of Darius the Persian, the son of Esther, who built the Temple. As for the three extra years, I do not know whether he started to fast before them or whether the duration of the acceptance of the vow of his fasts terminated three years after the building.