God created the world in six days and rested on the seventh. But He was not finished. Psalm 118 prophesies of another day (the 8th day) to come, “The stone that the builders rejected has become the cornerstone. This is the Lord’s doing and it is marvelous in our eyes. THIS IS THE DAY THAT THE LORD HAS MADE . . . v 22. This day ushered in the New Covenant that promised a circumcision of the heart, not made with human hands (Rom 2:29). I think God may have called for circumcision on the 8th day to point to Christ and the New Covenant “new creation” (Gal. 6:13-15) He would bring. After all, is not Jesus the point of everything? “For from Him and through Him and to Him are all things. To Him be glory forever. Amen.” Rom. 11:36
On the eighth day, the amount of vitamin K and prothrombin present is elevated above 100% of normal and is the only day in the males life in which this will be the case under normal conditions. If surgery is to be performed, day eight is the perfect day to do it. Vitamin K and prothrombin are at their peak on that day. Vitamin K and prothrombin are vital to coagulation, therefore stopping bleeding and healing faster. Ref. by S I McMillen, MD in his book "None Of These Diseases".
It seems that circumcision was a practice taken over from Egypt by the Israelites. In Egypt, grown men would circumcise themselves or get circumcised, and boys would do this on the border of manhood. This is common in many Ancient Near Eastern cultures. At least one of the goals of circumcision was to protect a person in the netherworld (Lods, A. 1943. "La 'mort des incirconcis'," Comptes rendues des séances de l'Académie des inscriptions et belles-lettres, pp. 271–283).
Although the scriptural sources suggest that 8-day circumcision had been practised for a long time (since Isaac) by the Israelites, that does not appear to have been the case. All these scriptural references are from the P-source (one of the sources of the Pentateuch), which was written in the context of the Babylonian exile in the 6th century BCE.
It seems then that we must explain both a late circumcision (your actual question) and an early circumcision (compared to related cultures). Propp (1987: "The Origins of Infant Circumcision in Israel," Hebrew Annual Review, pp. 355–370) suggests several reasons for an early circumcision on pp. 362–363:
[T]hrough experience the Israelites probably discovered the operation is less painful for babies. Infant circumcision also relieved the individual of the painful choice of undergoing the trauma, for in his adulthood he might have been reluctant. [...]
[C]circumcision may have been considered salutary. Either God pro-
tects the Israelite child, or demons avoid it [...]
Babies were circumcised as soon as it was safe because they were liable to die, for, as Lods (1943) shows, circumcision was believed to improve one's fate in the world of the dead.
And on p. 366 for a late circumcision:
[T]he blood clotting mechanism does not function fully until the age of six months and is particularly deficient in the first two to four days after birth. A few bad experiences would have shown the advisability of waiting a while. But why was the eighth day, i.e., a week plus a day, singled out as the proper time for circumcision? The number eight elsewhere signifies the end of a period of taboo. [...] It seems that the infant is taboo until its eighth day, when circumcision introduces it to the world.
(The part left out contains relevant references, but is omitted for brevity.)
I would like to add the following point to this: during the Babylonian exile, the Hebrews lived in a multicultural environment and felt it was important to establish an identity. It is from this that the strict rules from Leviticus and other P-passages originate. In such an environment, it can be important to "mark" a child as belonging to your group so that he remembers it himself, and feels physically connected to his own group.
There is a ritual purity problem with being in such close contact with inevitable blood and fluid discharges from the body of the woman when born. There are a number of blood-corpse-tomb purity laws in the Torah that were given to moses.
Short answer is because God gave the commandment:
The Lord spoke to Moses, saying: Speak to the people of Israel, saying: If a woman conceives and bears a male child, she shall be ceremonially unclean seven days; as at the time of her menstruation, she shall be unclean. On the eighth day the flesh of his foreskin shall be circumcised.
Generally contact with blood/semen/corpses/tombs creates ritual impurity that takes seven days (the number of completeness) to purify. The corpse/tomb impurity laws are in Numbers 19. Males can be pretty much clean of anything in seven days.
It may be helpful to compare this law to another similar law concerning sacrifices,
Do the same with your cattle and your sheep. Let them stay with their
mothers for seven days, but give them to me on the eighth day.
Some have regarded this seven-day period as a humane treatment of the animals, but i don't see it as such. The motive for the seven-day period is most probably a ritual concern than it is a purely ethical one; the ancients may have regarded young animals as ritually unfit for sacrifice similar to a blemished animal (perhaps it was considered too weak and unstable and incapable of surviving on its own). The number seven signified completion in the ancient world, so once the animal went through the seven-day period it was considered stable, healthy and fit for divine consumption.
Now you may ask, "what has this all got to do with circumcision?"
There is much evidence that circumcision in ancient Canaan was considered a kind of sacrifice to the gods, and it is reasonable to assume that the Israelites too regarded it as such. In fact there is evidence in the bible itself that circumcision was regarded as a kind of sacrifice. See Exodus 4:24-27 and also here for more on circumcision and its sacrificial nature. Hence the need to wait seven days before the child is fit for the circumcision ritual.
All God's commands are given for our protection or for our provision. He's not capricious nor does he do things without perfect reason and precision.
Two reasons that circumcision was commanded on the 8th day.
Several people have mentioned one aspect, but not the other.
One reason is physical or biological - relating to the body and how God created it to function.
the second reason is spiritual - conveys a deeper spiritual truth.
Medical doctors around the world as well as countless medical journals confirm that the bodies' natural blood clotting mechanism doesn't start until the 8th day.
Several people mentioned that doctors do it earlier all the time, all over the world.
This is true, but they give the child a shot that will activate or stimulate the blood clotting process.
God created the universe ex-nihilo - out of nothing, and he knew this long before "modern medicine" discovered that the blood clotting mechanism doesn't start until the 8th day.
"Dr S.I. McMillen, M.D., in his book None of These Diseases (2000) confirms that pain caused by circumcising later in life can last for up to a week. Newborns, on the other hand, are extremely resilient. So the most humane time is to circumcise in the first month of life. He also confirms that prothrombin only jumps to 110% of the adult level on the eighth day. Thus the safest day for circumcision in a baby's life is day eight. The fact that the Bible records the exact and earliest "cut off" point for a newborn reveals a divine fingerprint. God knew exactly what he was doing."
An article shared by famous surgeon and New York Times bestseller, Dr Joseph Mercola also states: ‘Day 8 is said to be the only time in a baby's life when his prothrombin level will naturally exceed 100 percent of normal.’
It is also very interesting that throughout the Bible, certain numbers have great Biblical significance.
It is well-known and accepted by Catholics and protestant Bible scholars for example that 3 is the number of the trinity, and the number 7 is the number of perfection and completeness.
Throughout the old and new testaments the number 40 is the number of testing -
Jesus tested in the wilderness 40 days and 40 nights,
Moses on the mountain 40 days and 40 nights,
Noah and the great flood - rained 40 days and 40 nights,
human pregnancy 40 weeks - number of testing combined with 7 number of perfection.
Children of Israel tested - wandered in the wilderness 40 years etc, etc.
This is not to be confused with numerology, which is evil and has to do with fortune-telling, or using numbers to be connected with someones destiny, much like astrology uses stars to predict or foretell someones destiny.
We also see over and over throughout scripture that 8 is the number of new beginnings.
The number 8 in the Bible represents a new beginning, meaning a
new order or creation, and man's true 'born again' event when he is
resurrected from the dead into eternal life.
Boys were to be circumcised on the 8th day. The number 8 symbolizes circumcision of the heart through Christ and the receiving of the Holy Spirit (Romans 2:28 - 29, Colossians 2:11 - 13).
Those in Christ are becoming a new creation, with godly character being created by the power of God's Spirit (2Corinthians 5:17,
Ephesians 2:10; 4:23 - 24).
God saved eight people on the ark in order to have a new beginning for mankind after the flood. Since the meaning of four is derived from
God's creation of everything, 8 (4 + 4) pictures the new creation
after the flood.
Jesus appeared 8 times after his resurrection.
Eight is the number of Jesus, whose name in the Greek adds up to 888.
Originally the milah(circumcision) ritual commanded by God in the old testament was a bloodletting ritual. The ritual required only the severing of the frontal part of the foreskin to scar it and let it bleed, not total amputation. Amputating the entirety of the foreskin would have killed too many children, even on the eighth day during this time period when survival beyond infancy was hard enough. Total amputation of the foreskin was not strictly required until later when Rabbi wanted to prevent Jewish men from being able to stretch out and restore the remnants of their foreskins. You can read more about that here in this book, page 44 and 45: https://books.google.com/books?id=SF6fbjNe0yYC&printsec=frontcover&dq=when+did+metzitzah+start&hl=en#v=onepage&q=milah&f=false