Michael Zaroovabeli July 18 at 3:28pm Reply • Report
The start of the ‘three weeks’ comes about on the 17th day in the Jewish month of Tammuz, Jews all over the world fast and lament to commemorate the many calamities that befell the Jews on this ominous day.

The 17th of Tammuz is quoted in the Book of Zechariah as "the fast of the fourth month.
A Mishna states that 5 tragic events occurred on this day.
Moses broke the two tablets of stone on Mount Sinai, The daily Tamid offering ceased to be brought on this day, The walls of Jerusalem were breached (proceeding to the destruction of the Temple); An idol was erected in the Temple. Also before the Bar Kokhba's revolt, the Roman military leader Apostomus burned a Torah scroll;

The breaching of the walls occurred on the 17th of Tammuz according to the Gemora during the end of both the first and second temple eras.

Other interesting occurrences on this day in Jewish history include; Noach sent out the first dove to see if the Flood waters had receded, as we learn in parshat Noach.
Moshe destroyed the Golden calf and ascended back up Mount Sinai for the second time where he spent the next forty days pleading for forgiveness for the sin of the golden calf, as we learned in parshat Ki Tisa.

This day is a sad day in history; a reason why we fast is to repent for the sins of them respective generations!

The fast of the Seventeenth of Tammuz is observed from the break of dawn until night (as defined by Jewish law), this fast is one of four Jewish fasts to be observed in this manner, the others being; 3rd Tishrei, 10th of Tevet, 13th Adar and 17th of Tammuz.