17 of Tamouz is a day of fasting in commemoration of many misfortunes.
The main reason for this day of fasting is the reparation, generation after generation, of the fault of the Golden Calf committed by the Bnei Israel, after their liberation from slavery.
Indeed, a few months after his departure from Egypt, the people who were waiting for the return of Moses on Mount Sinai to receive the Torah, had abandoned themselves to idolatrous impulses. When Moses saw what the people had done on that day of 17 Tamuz, the Tablets of the Law fell from his hands and broke.
This episode reflects the weakness of human trust. Of considerable gravity in Jewish history, it constitutes the act of absolute betrayal towards the Creator who had just taken the people under His protection.
Moses implored and obtained from God a day of atonement (Yom Kipur) and a second chance for the people. But on the 17th Tamouz remains a day of regret for this fault in particular, as well as for all the faults that the people commit in each epoch.
But other tragedies overwhelmed the Nation of Israel today:
The perpetual offering was discontinued for the first time in history during the siege of Jerusalem before the destruction of the First Temple.
A breach was pierced in the walls of Jerusalem, which led to the destruction of the Second Temple.
Apustamus, a Greek officer of the Second Temple period, burned all Sefarim in order to eradicate the Torah and its followers.
An idol was placed in the Sanctuary.
For these reasons, we observe a day of contrition, from dawn to dusk, without food or drink. This is not a day off: it is not forbidden to perform work.
References
Zechariah VIII, 19. “19 Thus saith Jehovah-Cebaoth, The fast of the fourth month, and the fast of the fifth, and the fast of the seventh, and the fast of the tenth month, shall be changed to the house of Judah, to joy and gladness, and to solemn feasts. But cherish truth and peace! “
Vayikra XVI, 30. “For in that day they will make atonement for you, to cleanse you: you will be cleansed from all your sins before the Lord. “