On a fundamental level, we circumcise a Jewish baby boy at eight days because that is what G‑d instructs us to do: “And on the eighth day, the flesh of his foreskin shall be circumcised.”1

Although the verse itself does not reveal why we are instructed to circumcise specifically on the eighth day, many possible explanations are offered:

The Power of Shabbat Queen

Having the circumcision at eight days guarantees that the baby will experience at least one Shabbat before the circumcision.

The Midrash explains that this is analogous to a king who decreed that any who wish to visit him must first pay their respects to the queen. Shabbat is commonly referred to by the sages and mystics as “the Shabbat queen,” and before entering the covenant with G‑d, the baby needs to first greet the Shabbat queen by experiencing the holiness of at least one Shabbat. This is also the reason why any offerings brought in the Temple needed to be at least eight days old.2

Guaranteeing at least one Shabbat also brings healing to the soul, which has just entered this physical, material world.3

Atonement

Others explain that just as the blood of the offering brings atonement, so too does circumcision. Therefore, just as an animal brought for an offering needs to be at least eight days old,4 at the time of the circumcision, the baby needs to be at least eight days old.5

Health of the Baby

Maimonides explains that we wait eight days so that the child will be strong enough for the circumcision.6

Parents Can Share in the Joy

The Talmud explains that since a woman is considered ritually impure for at least seven days after giving birth to a boy, during which time the couple cannot be physically intimate with each other, we wait eight days in order that the parents not be “mired in sadness” during the joyous occasion.7

Mourning Over Learning

The Talmud tells us that while the baby is in the mother's womb, he is taught the entire Torah. As he enters into the world, an angel causes him to forget all that he learned.8

Based on this, some explain that we wait eight days for the circumcision since in the first seven days the soul is mourning its loss.9

Natural vs. Supernatural

The brit milah (circumcision) is a sign of the eternal covenant and bond between the Jewish people and G‑d, our Creator. Our covenant is suprarational; it does not dissipate in moments when we don’t comprehend why we do what we do. Therefore, the brit milah is a matter of faith; it signifies a bond that is higher than intellect. It is for this reason that we don’t wait to circumcise a child until he is old enough to make his own decisions, but instead circumcise him when his relationship with G‑d transcends intellect.

And it is for this reason, the mystics explain, that the baby is specifically circumcised on the eighth day.

Since G‑d created the world in seven days, there are many matters of Torah and mitzvahs which reflect the number seven. There are seven weeks of the Counting of the Omer, seven years of the Shemittah (Sabbatical year) cycle, and seven shemittot of Yovel (the Jubilee).10 Thus, the natural world is represented by the number seven.

Eight, on the other hand, represents the suprarational and the infinite, that which is beyond the natural order of this world. Therefore, the child is circumcised specifically at eight days, for the child is entering a religion founded upon faith, whose survival is miraculous, and whose potential in the world is infinite.11

Ultimately, we don’t know the true reason why the brit milah is done specifically at eight days. In not telling us the reason, the Torah is showing us that just like the brit milah itself, the eighth day is suprarational.