For eight hundred and thirty years there stood an edifice upon a Jerusalem hilltop which served as the point of contact between heaven and earth. So central was this edifice to the relationship between man and G‑d that nearly two-thirds of the mitzvot are contingent upon its existence. Its destruction is regarded as the greatest tragedy of our history, and its rebuilding will mark the ultimate redemption-the restoration of harmony within G‑d's creation and between G‑d and His creation.

A full three weeks of our year--the three weeks "between the strictures"1 of Tammuz 17 and Av 9--are designated as a time of mourning over the destruction of the Holy Temple and the resultant galut--physical exile and spiritual displacement--in which we still find ourselves.

In this period, many calamities befell the Jewish people throughout the generations. It was during this period of between the straits that both the first and second Temples were destroyed.
During this period, we lessen the extent of our rejoicing. We don't:
Conduct weddings. (Engagement parties – without music – are allowed until Rosh Chodesh Av.)
Play musical instruments or listen to music.
Recite the Shehecheyanu blessing. Thus, we do not wear new clothing or eat fruit which we have not yet eaten this season so that we will not be required to recite Shehecheyanu.
Take a haircut or shave. Many Sephardic communities permit haircuts and shaving until the week of Tisha b'Av (in other words, until the Saturday night before the fast).
(Speak to your rabbi if there are extenuating circumstances that prevent you from observing any of the above customs.)