Any baby whose bris is in fact on the eighth day, has it then.
Shabbos, Yom Tov, Yom Kippur. A few exceptions, besides a bris which
is delayed for medical reasons, apply, so your LOR should be consulted.
My cousin in fact made a bris for his son this past Yom Kippur. The seuda
(celebratory feast) is obviously at night after the fast.
Gershon