- Peninei Halakha
The Sages state in the Mishna (Ta’anit 26b), “When Av arrives, we curtail our joy,” because this is a period of mourning over the Temple’s destruction. Therefore, one should not engage in joyous activities like hikes, hotel vacations, and social gatherings. Communities may schedule only events whose main purpose is educational or communal. In addition, one who needs to relax for health reasons may take a vacation in a hotel or a convalescent home (see above, section 6).
It is customary to refrain from arranging the warp strings (sheti) on a weaving loom during the Nine Days. The basis for this is the fact that the function of the Foundation Stone (even ha-shtiya), upon which the Holy of Holies stood and which is the foundation of the world, was abrogated. Therefore, we refrain from arranging the warp strings, which are the foundation of a garment. One also may not sew or knit new clothes or crochet a yarmulke during the Nine Days (sa 551:7-8). However, one may mend old clothes. One who makes a living sewing clothes or manufacturing material, and needs the income from the Nine Days, should ask a Torah scholar how to act.
It is best to cancel sewing lessons during the Nine Days, but if necessary, one may continue them, on condition that the students do not engage in sewing new garments. Rather, they should mend old ones or practice sewing on useless pieces of cloth.
The Sages also say that during this period, one should curtail business dealings that are joyous. It is even proper to decrease one’s involvement in all types of transactions. We also avoid joyous building projects and plantings; we will explain the details of these laws below (section 18). Since we curtail our joy during this period, it became customary to refrain from consuming meat and wine, which make one happy (as explained in sections 13-15).
Since this is a period of calamity, the Sages recommend that one who is involved in a legal dispute with a non-Jew should avoid going to court in the month of Av, because the Jewish people have bad fortune during this time and he is liable to lose the case (sa 551:1).
 The simple understanding is that one should avoid interacting with the non-Jew until Tisha Be-Av, but afterward one may contend with him as he would throughout the year. Korban Netanel quotes this interpretation in the name of the Zohar. ma, however, cites Rabbeinu Yeruĥam as saying that one should avoid interacting with the non-Jew throughout the month of Av. See mb 551:2, sht ad loc. 2.