Making a Bris Milah on the 10th of Teves:
The Baal Bris [father of the child, Mohel and Sandak] are all required to fast the entire day even though the Bris is taking place that day. Thus the Seudas Mitzvah is to take place only after the fast.
Who is considered a Baal Bris?
The Mohel, sandek and father [and mother] of the baby.
If a Pidyon Haben is set to take place on a fast day the father and Kohen must nevertheless fast until night as usual, and the meal is to take place at night, after the fast.
 M”A 559/11 regarding Tishe Beav and all the other 4 Taaneisim and so rules: Radbaz 2/35; M”B 559/35; Beis David 347; Erech Hashulchan 550/1; Poskim brought in Kaf Hachaim 549/5 and 10; See Piskeiy Teshuvos 559/9
 Rama 559/8
 Rama 559/8; P”M 559 A”A 12
 M”B 559/38; See Piskeiy Teshuvos 559/10