Bén hámécárim

There are those that are accustomed not to eat meat nor drink wine this week. (But, it is allowed to use wine vinegar) (Agudah and Maharil). And there are those that add (on the custom to not eat meat/drink wine) from Rosh Chodesh (Av) until the Fast (Tisha B'av), and there are those that add from Shiva Asar B'Tammuz. Rema We cover from Rosh Chodesh and on the slaughtering knife (R' Yeruchom Path 11 Chelek 3 and Haagos Maimoni) because we only ritually slaughter for a mitzvah need, for example for a sick person, Shabbos, a Bris Milah, a Pidyon Haben, or similar.

There are those that don't bathe from Rosh Chodesh (Av), and there are those that withhold from this week, and there are those from the 17th of Tammuz until Tisha B'av. Rema: Yet for a mitzvah need it is allowed; therefore, a Niddah can bath and immerse (in a mikvah) [Maharil]. Even one who is immersing on the night of the 10th of Av, she is permitted to bathe the eve of Tisha B'av if she cannot do so on the night of the 10th of Av [Agudah]. And it seems that so to a woman who will dress in whites (for her 7 days of purity) is able to wash a bit normally like the rest of the year - since it is not done for pleasure only for the sake of the mitzvah. The custom is to not wash even with cold (water) from Rosh Chodesh and on [Terumas HaDeshen Siman 106]. Even on the eve of Shabbos Chazon, it is forbidden to wash except the head, face, hands, and feet in cold water (Maharil and Teshuvas Maharil Siman 16 and Bais Yosef). Yet there are those are lenient in immersing the head in hot water to one who normally does so every week.

It is necessary to be careful from the 17th of Tammuz until the 9th of Av not to go out alone from the 4th to 9th hour (of the day) because the demon Meriri rules and do no hit students on these days.

From Rosh Chodesh until the Fast we minimize business transactions and building of joy, example the wedding house for his son or the building of molding and decorating (i.e. of walls and ceilings). Or joyous planting, example a royal banqueting canopy that is planted for shade to stand in its shade or types of myrtle and other tents. If its wall is leaning to fall even though it is of joy, it is permitted to build (and for the needs of a mitzvah, all is allowed) (Ran Last Chapter of Taanis) . We do not wed women and we not make engagement meals, but engaging without a meal is allowed. Even on Tisha B'av itself, it is permitted to engage that one not precede him. Rema: Our custom is to be strict that we do not wed from the 17th of Tammuz and on until after Tisha B'av (Minhagim).

After the fast, it is permitted to get a haircut, to launder immediately. If Tisha B'av falls out on Sunday or Shabbos and is pushed out off until after Shabbos, it is permitted for both weeks whether before the fast or after. There are those that say that the custom to forbid the whole week except for Thursday and Friday. Rema: We are accustomed to be strict from the beginning of Rosh Chodesh in the matter of laundering but haircuts we are accustomed to be strict from the 17th of Tammuz (Minhagim)

...

Judah has gone into exile
Because of misery and harsh oppression;
When she settled among the nations,
She found no rest;
All her pursuers overtook her
In the narrow places.^-b

Another matter, “All her pursuers have overtaken her within the straits,” in days of distress, between the seventeenth of Tamuz and the ninth of Av, during which Ketev Meriri is prevalent, just as it says: “Nor the pestilence that stalks in darkness, nor the destruction [ketev] that lays waste at noon” (Psalms 91:6).

Not only does one fast on the Ninth of Av, but from when the month of Av begins, one decreases acts of rejoicing. During the week in which the Ninth of Av occurs, it is prohibited to cut one’s hair and to launder clothes, but if the Ninth of Av occurs on a Friday, on Thursday these actions are permitted in deference to Shabbat. On the eve of the Ninth of Av a person may not eat two cooked dishes in one meal. Furthermore, one may neither eat meat nor drink wine. Rabban Shimon ben Gamliel says: One must adjust and decrease the amount he eats. Rabbi Yehuda obligates one to overturn the bed and sleep on the floor like one in a state of mourning, but the Rabbis did not agree with him.