The eighth day of a boy’s life is particularly suited to circumcision
In Leviticus 12:2-3 G-d tells Moses that a male child has to be circumcised on his eighth day. Rabbi Pinchas Taylor in Pillars of Faith states the following on page 187:
It has become more evident through medical research in the more recent generations that the eighth day is the earliest advisable time to perform a circumcision. The major clotting agents, prothrombin and vitamin K, do not reach peak levels in the blood until the eighth day of life. Vitamin K forms between the fifth and seventh day, and prothrombin levels … normal at birth, drop to very low levels in the next few days, and after the eighth day reach their average of 100%. Furthermore, on the eighth day – and only on the eighth day – prothrombin levels are at 110 percent, more than any other day in a boy’s entire life.
And he goes on to quote Dr Sim McMillen:
From the consideration of vitamin K and prothrombin determinations, the perfect day to perform a circumcision is on the eighth day.