The 17th of Tammuz is a fast day commemorating the fall of Jerusalem, prior to the destruction of the Holy Temple.
It is the first of four fast days mentioned in the Neviim. The purpose of a fast day is to awaken our sense of loss over the destroyed Beit Hamikdash and how we went into exile.
The Talmud says that after the future redemption of Israel and the rebuilding of the Beit Hamikdash, these fast days will be re-dedicated as days of rejoicing and festivity. For as the prophet Zechariah says: the 17th of Tammuz will become a day of "joy to the House of Judah, and gladness and cheerful feasts." (Zecharia 8:19)
In the First Temple Era, the Kohanim stopped offering the daily sacrifice on this day (Taanit 28b) due to the shortage of sheep during the siege and the next year 3184 (586 BCE), the walls of Jerusalem were breached after many months of siege by Nebuchadnezzar and his Babylonian forces.
King Menashe, one of the worst of the Jewish kings, had an idol placed in the Holy Sanctuary of the Temple, according to tradition on this date (Melachim II 21:7/ Sanhedrin 102). The Talmud further says (Taanit 28) that in the time of the Roman persecution, Apostomos, captain of the occupation forces, did the same, and publicly burned the Torah. These were followed by Titus and Rome breaching the walls of Jerusalem in during the end of the second Beit Hamikdash era.
The 17th Tammuz is a very well known day in the Jewish calendar. It is the start of the ‘Three weeks,’ that Jews refrain to have simchas, holidays, this time period is a time of morning about the destruction of the Temple, this is from 17th Tammuz through to Tishah B'Av, commemorating the period between the fall of Jerusalem and the Destruction of the Temple. The Jerusalem walls were once again breached on this day.
The 17th of Tammuz is also the day that Moshe came down from the heavenly realms with the two tablets, in anguish, while witnessing the Jews dancing round the Golden Calf, he smashed and broke the tablets on the floor, in turn the broken tablets resulted in their being added forgetfulness in the world (Eruvin 54a).
This Dvar Torah is dedicated to Michal Bat Chana who has had a problem with her memory, please everyone pray for her to get better!