Responsa על מועד קטן 44:4

A. A baal-berith (the person who holds the child during circumcision) is permitted to change his clothes and to bathe on the day the child is circumcised, even if such day be within his "period of mourning" (thirty days). The prohibition against changing one's clothes and bathing within the "period of-mourning" is based on custom and does not apply to persons performing a Mitzvah (religious duty). Within the thirty days of mourning, however, a person is not permitted to take part in a religious banquet, though one is permitted to return a visit to a secular feast. Therefore, the baal-berith is not permitted to take part in the banquet following circumcision.
SOURCES: L. 227, 8, 9; cf. Mord., Moed katan, 891.