The 17th of Tammuz has historically been a calamity prone day. The Mishna (Tanis 4:6) lists 5 things that happened to the Jewish nation n this day:
1) The Luchos were broken when Moshe Rabeinu descended from Har Sinai and came across the Golden Calf. (40 days after the 7th of Sivan).
2) The Korban Tamid (the twice daily sacrifice that was brought in the Beis Hamikdash) ceased [either due to government edict or because of internal Hasmonean conflict c.f Bava Kama 82b].
3) The walls of Jerusalem were breached during the destruction of the Second Bais Hamikdash according to the Babylonian Talmud, and during both destructions according to the Jerusalem Talmud.
4) Apustamus [a Greek minister] burnt the Torah. The Tiferes Yisroel understands this to mean the Torah that Ezra had written and was used to check other Torah scrolls against. Alternatively it refers to any and every Torah he could get his hands on.
5) An idol was erected in the Beis Hamikdash. When this occurred is a dispute in the Jerusalem Talmud. Some say it refers to the idol of Menashe during the first Temple period, and some say it was erected by the aforementioned Apustamus, which was during the Second Temple period.
For laws of fast days please see the article on Fast Days