Mourner:[1]

An Avel who is a Baal Bris [Father of son; Mohel; Sandek] may wear Shabbos clothing for the Bris, until after the Mila.[2] Some Poskim[3] rule that this applies even during Shiva. Other Poskim[4], 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[5] 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.[6]

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

[2] 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,

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

[4] 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

[5] 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

[6] 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]