9. Shabbos clothing during a Bris:[1]

It is accustomed that the father [and mother[2]] of the child, the Mohel and the Sandek [and Kevatrin[3] and grandparents[4]] to wear Shabbos clothing for the Bris. [Furthermore, the custom is for also other close relatives to wear Shabbos clothing.[5]]

Dressing the baby in elegant clothing:[6] In honor of the circumcision, the baby, who is the Chasan of the Bris, is to be dressed in elegant clothing similar to the day that he enters the chuppah.

Nine days and Tishe Beav: When a Bris is taking place during the nine days, it is accustomed for the father [and mother[7]] of the child, the Mohel and the Sandek [and grandparents[8]] to wear Shabbos clothing. [Furthermore, the custom is for also other close relatives to wear Shabbos clothing.[9]]

Tishe Beav:[10] The Baalei Habris may wear Shabbos clothing when a Bris takes place on Tishe Beav. The Shabbos clothing may be worn until after the Bris. However, they may not wear leather shoes.[11]

Mourner:[12] An Avel who is a Baal Bris [Father of son; Mohel; Sandek] may wear Shabbos clothing for the Bris, until after the Mila.[13] Some Poskim[14] rule that this applies even during Shiva. Other Poskim[15], however, rule that this applies only after the Shiva is over, during Shloshim, however during the Shiva they may not wear Shabbos clothing. Other Poskim[16] rule it applies only after the Shloshim, during the year of Aveilus. The above is only regarding Shabbos clothing, however, during Shiva they may not wear leather shoes.[17]

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[1] Michaber and Rama 551:1; Shevach Habris 17:7; See Encyclopedia Talmudit Vol. 4 Erech Bris Mila p. 248 footnotes 40-42

[2] Chayeh Adam 133:21; Kitzur SHU”A 122:15; Kaf Hachaim 551:7

[3] See M”A O.C. 551:3; Elya Raba 551:27; Beis Meir 551; Chayeh Adam 133:21; Kitzur SHU”A 122:15; M”B 551:3;  Shaar Hatziyon 551; Shevach Habris 16:5; See Encyclopedia Talmudit Vol. 4 Erech Bris Mila p. 248 footnote 42 that only the woman needs to wear Shabbos clothing

[4] Shaareiy Teshuvah 551:1

[5] Shaareiy Teshuvah 551:1; Ashel Avraham Butchach Tinyana 551

[6] See Mateh Moshe Seder Mila; Machzor Vitri 505; Even Ezra Shemos 4:5; Kelalei Hmaila p. 64; Koreis Habris 265:60; Bris Avos 7:2; Likkutei Sichos 22:58; Shevach Habris 18:1

[7] Chayeh Adam 133:21; Kitzur SHU”A 122:15; Kaf Hachaim 551:7

[8] Shaareiy Teshuvah 551:1

[9] Shaareiy Teshuvah 551:1; Ashel Avraham Butchach Tinyana 551

[10] Rama 559:8

[11] Bigdei Yesha , brought in Biur Halacha 559 “Umutar”

[12] See Pnei Baruch 20:3 and 14; Nitei Gavriel 23:25; 24:3 and 10 footnote 6

[13] Rama 391:2 “An Avel who is a Baal Bris or Mohel may wear Bigdei Shabbos until after the Mila”; Teshuvas Maharil 116 in name of Sefer Haparnes 240; Tashbeitz Katan 425; Shevus Yaakov 2:102 regarding Pidyon Haben; See Noda Beyehuda Tinyana Y.D. 213, brought in Pischeiy Teshuvah 391:6, who questions this ruling although concludes to follow Rama,

[14] One way of learning Rama 391:2; Pnei Baruch 20:3; Nitei Gavriel 24:3 footnote 6 in name of Poskim

[15] One way of learning Rama 391:2 and so learns Darkei Hachaim 14:8; Birkeiy Yosef 391:1 regarding Sandek; Elya Raba 132:4 that that all laws of Aveilus apply to him just like a Sandek; Darkei Hachaim 14:8 Biur 3 prohibits wearing within Shiva and says he may only wear freshly laundered clothing if another wears it first; Nitei Gavriel 23:21 in name of Maharil Milah; Yifei Nof 156; See Nitei Gavriel 23:25 and 24:10 and Pnei Baruch 20:14

[16] Derech Hachaim 263:5 and Kitzur SHU”A 212:3 in their way of learning Rama 391:2 that within Shloshim the Baalei Bris may not wear Shabbos clothing

[17] Birkeiy Yosef 391:1 regarding Sandek; Elya Raba 132:4 in name of Aguda and that that all laws of Aveilus apply to him just like a Sandek; Poskim brought in Pnei Baruch 20 footnote 6 and Nitei Gavriel 24:3 footnote 4 and so he concludes there to be stringent

Other opinions: Some Poskim rule the father is permitted to wear regular shoes. [Poskim brought in Nitei Gavriel 24:3 footnote 3]