The 17th of Tammuz is mentioned in Nevi'im (Prophets) - as "the fast of the fourth month" (Zechariah 8:19). The Mishnah (Taanit 4:8) lists five calamities that befell the Jewish people on this date:
Moses broke the two tablets of stone on Mount Sinai;
The daily tamid offering ceased to be brought;
The walls of Jerusalem were breached (proceeding to the destruction of the Temple);
Prior to Bar Kokhba's revolt, Roman military leader Apostomus burned a Torah scroll;
An idol was erected in the Temple.
The Babylonian Talmud (Taanit 28b) places the second and fifth tragedies in the First Temple, while dating the third tragedy (breach of Jerusalem) to the Second Temple period. Jerusalem of the First Temple, on the other hand, was breached on the 9th of Tammuz (cf. Jeremiah 52.6-7).
The Seventeenth of Tammuz is the second of the four fasts commemorating the destruction of the Temple and the Jewish exile. It is preceded seven months by the fast of the Tenth of Tevet and arrives three weeks prior to the full-day fast of the Ninth of Av. The last of the four fasts is the Fast of Gedalia, which is observed on the third or fourth day of Tishri.
http://en.wikipedia.org/wiki/Seventeenth_of_Tammuz