Q. If someone is invited to a Bris Milah or a Bar Mitzva during the Nine Days, can one wear Shabbos clothing?
A. Horav Shlomo Miller’s Shlit’a opinion is that is is permitted.
Piskei Teshuvos (559: 8) permits the parents of the child on a Bris on Tisha Beav, to wear Shabbos clothing, but not the grandparents etc.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit’a