This Halacha is an excerpt from our Sefer
Wearing new clothing:
It is forbidden to wear new clothing, whether white or colored, whether of wool or linen, during the nine days. This prohibition applies even on Shabbos. This prohibition applies even against wearing new shoes [and non-important garments, such as undergarments, socks, undershirts and underwear]. [From Rosh Chodesh Av until the week of Tisha B’av this matter is forbidden due to a stringency, while during the week of Tisha B’av it is forbidden from the letter of the law.]
May one wear new clothing on Shabbos which falls during the nine days?
Some Poskim rule it is forbidden to wear new clothing on Shabbos, [even if one is accustomed to wear Shabbos clothing on Shabbos]. It is even forbidden to wear new shoes and socks. Other Poskim however rule that those who wear Shabbos clothing are also permitted to wear new clothing for Shabbos [if he does not have other clean Shabbos clothing available] so long as the clothing is not of importance to require Shehechiyanu. If, however, the clothing is expensive and important, and requires Shehechiyanu, then it may not be worn on Shabbos.
A Chasan: A Chasan who is accustomed to wear a Shtreimal on the Shabbos prior to his wedding, may do so on Shabbos Chazon. However, whether he may wear other new clothing is subject to the same dispute as that mentioned above, of which even according to the lenient opinion, one may only wear new unimportant clothing. Some Poskim however defend the practice of Chasanim wearing all new clothing on Shabbos Chazon.
May a Bar Mitzvah Bochur wear new clothing on his Bar Mitzvah?
He may not wear new clothing on the day of his Bar Mitzvah. However, some Poskim rule that he may wear the new clothing on the Shabbos before he becomes Bar Mitzvah, even if it is Shabbos Chazon, and he is then permitted to wear it on his Bar Mitzvah, even during the nine days.
For how long is a clothing defined as new?
Once a clothing has been worn even one time, it is no longer defined as new and may be worn during the nine days.
May one buy and wear a new Tallis during the nine days?
It is permitted to buy a Tallis Katan up until the nine days. One however may not purchase a Tallis Gadol throughout the three weeks. If, however, one’s current Tallis Gadol/Katan is Pasul, and he does not have another one available, and is unable to borrow one in its place, he may purchase and wear a new Tallis Katan/Gadol.
May wear new non-leather shoes on Tisha B’av?
Yes. If, however, one has other non-leather shoes available, it is best not to wear new ones. Likewise, if the shoes were purchased prior to the nine days, one is to wear the shoes for the first time, prior to the nine days.
 Michaber 551:6 regarding week of Tisha B’av and Rama 551:6 that we are Machmir from Rosh Chodesh; Tur in name of Ramban
Ruling of Michaber and Sefaradim: The Michaber rules it is only forbidden to wear new clothing during the week of Tisha B’av. [Michaber 551:6; Beis Yosef 551; M”A 551:23; Levushei Serud 551; Mateh Yehuda 551:6; Machatzis Hashekel 551:23; P”M 551 A”A 23Kaf Hachaim 551:98]
 M”A 551:21; Elya Raba 551:40; M”B 551:9; Kaf Hachaim 551:16; See also Zera Emes 76; Machazik Bracha 551:3; Zechor Leavraham 551:9; Shaareiy Teshuvah 551:1; Kaf Hachaim 551:9
 M”A 551:22; Aguda; Hagahos Maimanis; M”B 551:47; Kaf Hachaim 551:95
 Kaf Hachaim 551:88; Piskeiy Teshuvos 551:27
 See Michaber and Rama 551:6
 Piskeiy Teshuvos 551:27
 M”A 551:6 in name of Minchas Kohen regarding a Chasan who is allowed to wear Shabbos clothing on Shabbos Chazon; M”B 551:9 [unlike his ruling in Biur Halacha 551:6]; Shaareiy Teshuvah 551:2
 So is proven from M”A ibid who rules that a Chasan may not wear new Shabbos clothing, even though he may wear Shabbos clothing
 Shaareiy Teshuvah ibid; See Piskeiy Teshuvos 551:26
 Chayeh Adam 43:14; Kaf Hachaim 551:89; See P”M 551 A”A 20 and Biur Halacha 551:6 “Keilim” regarding a Chaluk that just like he may launder the Chaluk for Shabbos, so too he may wear a new Chaluk. [Vetzaruch Iyun how this does not contradict the M”B earlier ruling in 551:9! See Piskeiy Teshuvos 551 footnote 142
 Igros Moshe 3:80; Piskeiy Teshuvos ibid
 Chayeh Adam 43:14; Kaf Hachaim 551:89
 Darkei Chaim Vehsalom 945; Shulchan Haezer 2:13; Piskeiy Teshuvos 551:5
The reason: As otherwise it is considered public Aveilus. [ibid]
 Darkei Chaim Veshalom ibid
 M”A 551:6 regarding Chasan
 Nitei Gavriel 4; Piskeiy Teshuvos 551:5
 Mateh Yehuda 551; Kaf Hachaim 551:90
 Igros Moshe 3:80
 As Tefillin are needed only for the Mitzvah, and are hence not included within the prohibition. A Tallis Katan is also permitted as it is not considered an important clothing to require Shehechiyanu.
 Sheilas Yaavetz 82; Pischei Olam 23; Kaf Hachaim 551:92
 Halachos Ketanos 2:139; Zechor Leavraham 9; Moed Lekol Chaiy 10:18 and 37; Ruach Chaim 551:3; Ben Ish Chaiy Devarim 9; Kaf Hachaim 551:96-97; Igros Moshe 3:80
 The reason: As since the entire purpose of the shoe is to commemorate the mourning, it is therefore permitted to be purchased and worn. [ibid]