The 17th day of Hebrew month of Tammuz is a date on which terrible national tragedies happened to the Jewish People. Therefore, we fast during the daytime on that date (see “The Purpose of Fasting”).
We fast because of five things that occurred on this day: The First Tablets were broken, the daily offering ceased, the walls of Jerusalem were breached, the evil Apostumus burnt a Torah scroll and an idol was set up in the Sanctuary.
The 17th of Tammuz occurs exactly three weeks before Tisha B'Av and begins a three week period of mourning for the destruction of the Beit Hamikdash.
