There are four fast days that have been established based on prophetic writings, after the destruction on the first temple. Each fast commemorates a different stage of destruction of the Judaic kingdom and the temple.

 ~ The Tenth of Tevet is the day that Nebuchadnezzar began his siege on Jerusalem
 ~ The Seventeenth of Tammuz is the day the Babylonians breached the walls of Jerusalem
 ~ The Ninth of Av is the most severe of the four fast days since on that day both the first and second temple were destroyed
 ~ The Third of Tishrei is also called The Fast of Gedalia is the day that Gedalia Ben Achikam - the leader of those remaining in Jerusalem after the Babylonian exile – was murdered. His demise marked the final destruction of the Jewish settlement in Jerusalem during the period of the first Temple.

The walls of the first temple were breached on the ninth day of Tammuz according to Jeremiah 39:2 and 52:6-7. This day was then marked as a day of fasting and mourning however it was changed to a festive day when the second temple was built. When the wall of Jerusalem were breached again, before the destruction of the second temple, this time on the seventeenth of Tammuz, rabbinic sages changed the date of the fast from the ninth to the seventeenth of Tammuz.
 

Mazor Guide to Shiva-ah Asar b'Tamuz (17th Day of Tamuz) brings you much more about the holiday, its meaning and its traditions... See the links below.

• Shiva-ah Asar b'Tamuz - 17th Day of Tamuz General Information
• Fast Days Commemorating Destruction of Temple
• 17th Day of Tamuz - a Day of Calamities
• 17th Day of Tamuz - a Day of Joy?
• The Three Weeks of Mourning Commence