Nine days and Tishe Beav:

When a Bris is taking place during the nine days, it is accustomed for the father [and mother[1]] of the child, the Mohel and the Sandek [and grandparents[2]] to wear Shabbos clothing. [Furthermore, the custom is for also other close relatives to wear Shabbos clothing.[3]]

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

[2] Shaareiy Teshuvah 551:1

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