What happened on the 17 of Tammuz to inspire the fast observed on that day?
- Moshe Rabbeinu broke the luchos when he came down from Har Sinai and saw the eigel
- The karbon tamid was no longer brought in the first Bais Hamikdosh.
- The destruction of the second Bais Hamikdosh started.
- Apostumos burnt the Torah
- A tzelem was placed in the heichal
Source: Mishna Ta'anis 4:6