The following halachot are meant for the period between the evening of the 17th of Tammuz and until Rosh Chodesh Av. From Rosh Chodesh Av the intensity of mourning increases and additional forms of simcha are prohibited.
During the "three weeks " many forms of simcha (rejoicing) are prohibited, including: weddings, music and dancing, haircutting, and reciting " she'hechiyanu ."
Weddings:
While there is no exception to the prohibition of weddings during the "three weeks," engagements/tenaim may take place, even if a meal is served as part of the celebration [Mishna Berurah 551 note 16]. However, once the "nine days" begin, engagement meals are forbidden.
Dancing and Music
According to most authorities, there is no difference between live and recorded music and both should be prohibited during the "three weeks" [Shulchan Aruch, Orech Chayim 551:2; Magen Avraham, ibid., note 10]. However, Shearim Mitzuyanim B'halacha does permit recorded music [Kuntres Acharon 122:2]. (In fact, some even forbid listening to acapella music suggesting that the equipment used to play this music has the status of a musical instrument [Shevet Halevi 2:57 and 8:127].).
Nevertheless, there are some who do permit recorded “serious music” during the three weeks [e.g. Rabbi Z. N. Goldberg, Binyan Ariel, OC 8 p.63; HaRav Nachum Rabinovitch [Siach Nachum Orech Chaim 35], HaRav Shlomo Dichovsky [Techumin 21] and HaRav Eliyahu Schlezinger [Eleh Hem Moadai 3:63].
A professional musician may accept jobs to play for non-Jews until the beginning of the "nine days" and practicing music is permitted until then as well [Shearim Mitzuyanim B'halacha, ibid.].
During the "three weeks" (and until the nine days) a person may attend a sporting event, even if background music is played [Moadei Yeshurun p. 151, note 16].
She'hechiyanu
Because this is a period of national mourning, the custom is to refrain from doing anything that would lead to the bracha of "she'hechiyanu." (After all, how can we make a "bracha" and thank G-d for "bring us to this time" - if it is such a terrible time for our people!) However, the Vilna Gaon, among others, disagreed with this custom [Mishna Berurah 551 note 98]. Therefore, during the "three weeks" the blessing of "she'hechiyanu" may be recited on Shabbat, or in a situation where an opportunity to purchase something would be lost (or even a special sale may end), if not purchased during the "three weeks" [Nitei Gavriel Hilchot Bein HaMeitzarim 17:9; Responsa Kinyan Torah 1:109].
Clothing that does not require a bracha of shehecheyanu may be purchased until Rosh Chodosh, therefore, one may purchase socks, shirt, shoes, ties, dishes or any other item [Mishna Berura, ibid. note 45]. If there is a big sale than one may purchase the garment although they require that one recite a shehecheyanu, however, it may not be worn until after Tisha B'Av [Mishna Berura ibid., note 11; Shaar Hatziyun note 12; Kaf HaChaim note 12].
One should refrain from buying a house during the three weeks unless one is in desperate need [Revivot Efraim 2:155 and 3:341; Nitai Gavriel 7;9]. Similarly, one should not move into a new house during the three weeks. If one cannot wait, than one should put up mezuzot and take some of belongings into the apt or house before the seventeenth of Tammuz [Levush Mordachai 1:101].
Haircuts and Shaving
The custom is to prohibit haircuts from the beginning of the "three weeks" until after the 9th of Av [Shulchan Aruch, ibid. 551:3]. Most authorities maintain that this even applies to children [ibid. 551:14; Mishna Berurah ibid. notes 81-82]. However, some do permit children's haircuts until the "nine days" [Mishna Berurah, ibid.]
For those who shave daily, there are three major opinions regarding shaving during the "three weeks":
1. It is prohibited to shave during the "three weeks" unless if a person must shave for business purposes [Igrot Moshe Choshen Mishpat 1:93].
2. It is also permitted to shave on Fridays in preparation for Shabbat [Shearim Ha Mitzuyanim B'halacha ibid. note 5].
3. It is permitted to shave until the beginning of the "nine days" [Nefesh HaRav p. 191].
