Over the centuries, several terrible events happened on the seventeenth day of Tammuz.
Five disasters took place on the 17th day of Tammuz.
1) The first of the Two Tablets containing the Ten Commandments were broken when Moshe Rabbeinu (our Teacher Moses) came down from Mount Sinai. This too place the year we left Egypt, in the year 2448 after Creation (1312 BCE). Moshe Rabbeinu came down and saw the Israelites worshiping the golden calf, and he attempted to absolve them of guilt by destroying the Two Tablets.
2) Menashe the son of Hezekiah erected an idol in the Holy Temple. This took place somewhere around the year 3280 After Creation (481 B.C.E.).
3) The Korbanai Tamid (the two Daily Sacrifices offered at the Holy Temple) were stopped and couldn’t be continued. This took place during the siege of the First Holy Temple (probably during the year 3338). This alone was enough of a calamity to cause a fast day to be instituted.
4) A wicked man by the name of Apostumos publicly burned the Torah. The details of this event are shrouded in history, since the Rabbis did not elaborate much about it, but some believe this took place around the year 3610 After Creation (151 B.C.E.).
5) The enemy penetrated the walls of Jerusalem prior to the Destruction of the Second Holy Temple, around the year 3828 After Creation (68 CE — some say 70 CE). This began the worst part of the Destruction.
All of these fives events occurred on the 17th day of the month of Tammuz.
The 17th day of the month of Tammuz was therefore ordained as a fast day. On the 17th of Tammuz we may not eat or drink, nor attend festivities, listen to music, take haircuts, nor buy or don new clothing. All this, including the fasting itself, begins in the morning of the 17th day of Tammuz.
The primary (but not sole) purpose of a fast day is to stir our hearts to repentance, and to serve as a reminder to us of our own actions and those of our forefathers. These sins are responsible for the terrible events that befell our people. Remembering these tragedies should lead us to examine our conduct and return to Hashem.
Fasting alone is not the point. Fasting is important for us, but changing ourselves is the way to get Hashem to take notice and change things. The only real way to change the world is to change ourselves.
The 17th day of Tammuz also marks the beginning of what we call ”the Three Weeks, a period during which we observe several types of mourning because of the events that happened to us during that time.
When the 17th day of Tammuz falls out on Shabbos we fast the next day, on Sunday, the 18th day of Tammuz.
About the name “Tammuz.” It seems odd that a month in the Jewish calendar would have the name of a Sumerian/Babylonian idol. But there is a history to it.
There was a brief period when a minority of people from some of the exiled Ten Tribes repented and were returned to the Land of Israel, during the time of the First Holy Temple. They wanted to constantly remember to thank Hashem for returning them to the Land of Israel, so they decided to institute a reminder. They therefore decided to rename the months of the Jewish calendar, using the names of the months that the Assyrians used for their calendar. Now we use those names in our calendar, instead of their original names.
I have heard that the Ramban (Nachmanides, 1194-1270) writes that when the Messiah comes, and our exile will end, we will use the names of the months of the civil calendar now in use (January, February, etc.), and apply them to the months of the Jewish calendar, as a reminder that we must thank Hashem for rescuing us from exile and recreating the Kingdom of Israel and the Holy Temple.
May it be soon!