The Fast of the 17th of Tammuz commemorates 5 particular tragedies according to the Mishnah:
- Moshe broke the first tablets when he descended Mount Sinai and saw the Golden Calf
- Offerings in the first Beis HaMikdash were suspended while Jerusalem was under attack
- The walls of Jerusalem were breached in the time of the second Beis HaMikdash
- Apustamus the Wicked burned the Torah [in the time of the second Beis HaMikdash]
- An idol was placed in the Sanctuary of the second Beis HaMikdash
In the larger picture, the Fast of the 17th of Tammuz marks the beginning of the Three Weeks, which we will talk about tomorrow! It culminates in the Tisha B'Av (the 9th of Av).