I should add that the mourning customs are not observed on Shabbat...
http://www.chabad.org/library/article_cdo/aid/144573/jewish/Three-Weeks.htmShabbatAll the laws of mourning are suspended on Shabbat.2 This includes the 17th of Tammuz and the 9th of Av3 themselves—when they fall on Shabbat, the fast days are postponed until Sunday, and the Shabbat is joyously celebrated. The Rebbe stated on numerous occasions that on these Shabbats we must increase our joy, and add an extra tasty dish to our meals, to emphasize that we are not in a state of mourning.
On the first Shabbat of the Three Weeks (or on the 17th of Tammuz itself, if it falls on Shabbat) we always read the Torah portion of Pinchas. The end of the reading discusses the three Jewish festivals. It has been said amongst chassidim that this alludes to the idea that the three Shabbats of the Three Weeks are to be celebrated on par with the three festivals...
Why is all mourning suspended on Shabbat?
Though these days and weeks heralded an exile rife with persecution and spiritual estrangement, it is our belief that ultimately this is for the good. Very soon, with the coming of Moshiach, we will understand that all the suffering was necessary in order to reach the ultimate good. At that time, the prophets foretell, these sorrowful days will be transformed into days of joy (see Breaking Walls for more on this topic).
Every Shabbat constitutes a foretaste of the Messianic Era. As such, on Shabbat we only focus on the positive element of this period.