Is the eighth day a requirement or just a custom?
The Mitzvah of Bris Milah begins on the eighth day of a male infant’s life. That being the case, a Bris done before the eighth day is halachically a meaningless procedure and therefore mandates another Bris. As stated explicitly in the Torah: "ובן שמנת ימים ימול לכם כל זכר" , at the age of eight days every male among you shall be circumcised . (Interesting to note, prothrombin and vitamin K, do not reach peak levels in the blood until the eighth day of life. Prothrombin levels are normal at birth, drop to very low levels in the next few days, and return to normal at the end of the first week. One study showed that by the eighth day, prothrombin levels reach 110 percent of normal. This may be one reason why the biblical commandment of Bris Milah only begins on the eighth day and doesn’t fulfill its obligation if performed before then. Therefore the Bris must be done on the eighth day of the child’s life, and must be done even on the Shabbos or jewish festivals including Yom Kippur when the eighth day of the babys’ life falls out on Shabbos or any other Jewish festival. However this only applies when your baby was born naturally and healthy. In the eventuality that your baby was born by caesarean section, the Bris would not take place on Shabbos or a Yom Tov, but postponed for the following day. In the event that your baby is not in perfect health e.g.; has a high fever, an eye infection, or most commonly, newborn jaundice, the Bris must be postponed until your baby’s physician and the Mohel agree to the healthy status of your child. Additionally, the Bris should be performed in the morning as the Torah states regarding Avraham Avinu "וישבם אברהם בבקר" – and Avraham woke up early in the morning to do this great Mitzvah of Bris Milah.