Brit Milah, often called "a bris," is one of the fundamental mitzvot of Judaism. It goes back to the start of the Jewish people where, as we read in Genesis 17, God commands Abraham to circumcise both himself, and then the male children who will be born to him, at the age of eight days. For Abraham to do that upon himself, especially with the primitive instruments of his day, was a deep act of faith. To do the mitzvah on one's male child today reenacts that same act of faith and trust in God. Although one can accept a little hyperbole in the statement, the Shulchan Aruch, the traditional Code of Jewish Law, starts its section on Brit milah with the statement that this mitzvah is more important than any other positive commandment of Jewish law.
Brit milah is normally carried out on the eighth day of the child's life (any part of a day is counted as the first day, which, following the Jewish pattern, begins anew each sunset.) It can NEVER be earlier. From a medical perspective, the coagulative strength of his blood (Vitamin K - prothrombin count) is higher than immediately after birth, yet his nervous system has not developed to the extent that he can localize pain sensations. The more important religious meaning, however, of the timing is that it is a commandment from God, going back to Abraham's circumcision of Isaac at the age of eight days. We read this each year as the Torah reading of the first day of Rosh Hashanah. At the age of eight days, the baby will perforce have lived through at least one Shabbat. Since Shabbat and Brit Milah are the two signs of the covenant, we can say that, as he has now experienced one sign, he is now "ready" for the other. If there are any medical reasons why the Brit milah should be postponed, halacha mandates that it MUST be postponed. When the eighth day would be Shabbat, that question is addressed separately.
The word "Brit" means "covenant." A covenant is a binding, unbreakable relationship of love and concern between two parties. The Brit, then, in the organ that takes part in the continuation of life, symbolizes the transmission of the covenant from one generation to the next. It links the newborn son to his people and, since in the course of the ceremony he formally receives his Hebrew name, it gives him his identity within the larger Jewish community.
There has always been conflict with segments of the larger society about circumcision/ Brit milah. The ancient Greeks forbad Brit milah, as it violated their idea of the holiness of the body. This was one of the causes of the revolt of the Macabees and the resulting Hanukah holiday. Today there are groups who question the medical necessity of circumcision. The American Academy of Pediatrics "flip-flopped" once again in the spring of 1999, and is now formally stating that there is no clear medical advantage to circumcision. However, many studies indicate numerous advantages for both children and adults. Source references are available upon request. One book summarizing the medical reasons for circumcision is Circumcision: A Parent's Decision for Life," written by Dr. Aaron Fink, (Kavanah Books, 1988).