Rosh Chodesh Av is on Friday, August 2, 2019. The Talmud (Ta'anit 26b) teaches that from the onset of the month of Av joyfulness should be lessened, and one who has a court-case with a non-Jew should postpone it until after the tenth of Av.
Many mourning customs evolved through time starting the Seventeenth of Tamuz leading to Tish'ah B'Av. These customs are part of our Jewish identity and it is important that each person keep his custom.
Sepharadim do not perform weddings from Rosh Chodesh (the new month) to the Ninth of Av.
Negotiations for and acquisitions of items for joyous purposes, such as marriages, are postponed until after the Ninth of Av. However, they are permitted if the items would not be available later, or if they would then be more costly.
Even though the Mishnah (Ta'anit 26b) restricts eating meat and drinking only during the Seudah Hamafseket, the last meal before the fast starts, many of the Rishonim rule that the prohibition starts from Rosh Chodesh Av until the tenth of Av midday since the Beth Hamikdash continued burning on the tenth of Av as well.
The Sephardic custom is to eat meat and drink wine on the day of Rosh Chodesh and Ashkenazim do not. The Yemenite custom is to eat meat until the Seudah Hamafseket as the Talmud rules. According to most opinions, meat and wine may be consumed on Shabbat since we do not mourn on Shabbat.
An Ashkenazi custom is not to bathe. However, according to many contemporary opinions, bathing in our day and age is part of daily hygiene and therefore is permitted. However, in order to fulfill the custom of mourning, bathing should be done in a different way (i.e. to shower with warm water instead of hot water). Separdim are allowed to bathe in hot water as usual.
According to many opinions, the blessing of Sheheheyanu should not be recited during this period of time since it is not an auspicious time. According to most opinions, the blessing may be recited on Shabbat since there is no mourning on Shabbat.
One should not listen to music.