With Rosh Chodesh Av, the more intense period of mourning for the destruction of the Beit Hamikdash, known as The Nine Days, begins.
“As Av enters, we diminish our joy.” This statement of the mishna is manifest in many laws & customs observed during the first nine days of the month of Av (Actually, it is for nine and a half to ten days).
There is a dispute among authorities as to whether this means that one ceases all joy during this period, or whether one is only required to diminish joy.
These reductions in joy manifest in numerous ways, some of them involving how we eat and drink. Things to be avoided at this time include listening to or playing music, giving or receiving gifts, laundering clothes, purchasing new clothes, cutting hair and shaving (even for those who otherwise do so during the rest of the Three Weeks), weddings and other festive gatherings, and bathing for pleasure. (Even when bathing for hygiene, one should take a colder shower than usual.)
When it comes to eating and drinking during the Nine Days, meat (including poultry) and wine are prohibited except on Shabbat. Meat and wine are associated both with joy. The Talmud in Pesachim (109a) bases this conclusion upon two verses: “You shall sacrifice peace-offerings and eat them there, and you will be joyful before the Hashem your God” (Deut. 27:7) and “Wine will rejoice the heart of man” (Psalms 104:15).
Additionally, meat and wine are both associated with the Temple service. Following the destruction of the Temple, some people wanted to impose upon themselves a perpetual ban on meat and wine, as these things were offered on the altar in the form of sacrifices and libations. Rabbi Yehoshua, considering such a move to be too extreme and unsustainable, encouraged them to mourn in a more realistic fashion (Baba Basra 60b). Abstention from meat and wine for a week is certainly a more achievable course of action than to do so in perpetuity.
While the Sages did generally prohibit wine and meat at this time, they did not include a meal celebrating a mitzvah, such as a bris (circumcision), a pidyon haben (redemption of a firstborn son) or a siyum (the conclusion of a tractate or other volume of Torah).
Even though havdala is officially after Shabbat, one is permitted to drink wine. It is preferable to give the wine to a child who is old enough to understand brachot (blessings) but not yet old enough to understand the concept of “mourning for Jerusalem.” Alternately, some authorities recommend the use of a substitute beverage for havdala such as fruit juice, beer, etc. Other authorities insist on wine as usual.