The Importance of the Eighth Day for Circumcision
(Excerpts from an article by Rabbi Lebovics´ son, Michoel Lebovics.)
Almost without exception, the bris must be done on eight day of the baby's life. What is the significance of the eighth day?
The world was created in seven days. Beyond the physical world - beyond seven days - is the eighth day, which represents the metaphysical, the spiritual. Since bris symbolizes taking the physical and elevatiing it to serve G-d, bris is done on the eighth day, a day that represents a level beyond physicality. Similarly, another explanation is that the number eight is composed of seven and one. Seven represents the world, which was created in seven days. One represents the one G-d who created this world. Eight, therefore symbolizes G-d's absolute sovereignty over His world, and our service to Him. It is befitting then that the bris milah a commandment which reminds us that life's purpose is to use the physical gifts that G-d gave us to serve Him, is performed on the eighth day.
The number eight appears in many places in Judaism. Hannukah is a celebration of the Jewish Peoples victory over the Greeks, a nation whose ultimate goal was to extinguish spirituality. It is no wonder that the oil in the Menorah miraculously burned for eight days, since the number eight represents spirituality. Other examples of significant eights include the eight special garments the kohen gadol (High Priest) wore when serving in the Temple and the eight strands of the four corners attached to the corners of the Tallis (prayer shawl).
Or Hachaim suggests another reason as to why the bris is on the eighth day. This assures that the baby has lived through one Shabbos (Sabbath). Once the baby has experienced the elevation from the secular weekday to the holiness of Shabbos, he is ready for the elevation of his physical body, the Bris.
Having the bris on the eighth day is so important that it even overrides the Shabbos. "Cutting" is forbidden on the Shabbos, yet if the eighth day falls on Shabbos, the bris is still done.
A notable exception to the "eighth day" rule is when a baby is ill. Often newborns are jaundiced and the bris is postponed until the baby is healthy. My father makes frequent house calls a few days before the bris to see whether the baby is "yellow" (Jaundiced babies usually have a yellow tinge to their skin that an expert can recognize.) After twenty years of checking babies, my father is as proficient in seeing whether a baby is jaundiced as most doctors.
As soon as the baby becomes healthy enough for the bris, it is done as soon as possible because one should be quick to perform a mitzvah. (However, once the eighth day has passed, the bris cannot be done on Shabbos. Only a bris taking place on the eighth day can override the Shabbos.)