Kapłańska 12:3 Komentarz: Sforno, Or HaChaim, Rabbeinu Bahya & Rav Hirsch

וביום השמיני ימול, for by that time the blood of his mother has congealed [dry blood does not confer impurity. Ed.] As a result the baby has become ritually cleansed, no moist blood being attached to it.

וביום השמיני, and on the eighth day, etc. Why did the Torah have to instruct us to circumcise a male baby on the eighth day, something that has been legislated already in Genesis 17,12 prior to the birth of Isaac, even? If all the Torah wanted was to tell us that the rite of circumcision must be performed by day and not by night, or that the importance of circumcising the baby on the eighth day is so important that the Sabbath may be violated in order to fulfil this commandment on time (compare Shabbat 132), why did G'd not include this information in Genesis?

וביום השמיני ימול, “and on the eighth day he shall be circumcised.” The word ביום means that even if that day happens to be a Sabbath circumcision is to be performed (Shabbat 131). This holds true only when one can be absolutely certain that the Sabbath is indeed the eighth day after the baby’s birth. When there is any doubt about this, such as the baby being born after sunset but before dark, the circumcision is postponed as we do no desecrate the Sabbath unless we are sure thereby to comply with the Torah’s instructions. The Talmud Shabbat 137 states that an infant may legally be circumcised (earliest opportunity) either on the eighth, the ninth, the tenth, the eleventh or the twelfth day of his life. He must not be circumcised sooner, and unless sick and weak not later (on pain of neglecting performance of a positive commandment incumbent on the father). If the baby has been born clearly during the day or night and is healthy he is to be circumcised on the eighth day. If he was born during dusk he is to be circumcised one day later providing that day is neither Sabbath or festival. If he is born at dusk on Friday he is to be circumcised on Sunday, i.e. the tenth day. If the Sabbath is followed by a festival such as Passover or Tabernacles his circumcision takes place on Monday, i.e. the eleventh day. If the Sabbath is followed by New Year, he is circumcised on the twelfth day of his life, i.e. on the Tuesday following New Year. If the baby was sick, circumcision is postponed until after he is well again.