The birth of a male child is followed by the rite of Brit Milah (circumcision). The rite dates back to the times of Abraham, who, as a sign of his covenant with G-d, was commanded to circumcise himself and his sons Ishmael and Isaac.
Given that Isaac was eight days old at the time of his circumcision, today the brit milah is performed on infants who have reached their eighth day of life. The ceremony may be postponed for health reasons.
Even though, in theory, the obligation to circumcise his son falls upon the father, in practice, the circumcision is performed by an expert called a mohel. A special honour surrounds the sandek, that is, the person who holds the child in their arms during the rite.
This honour is usually given to grandparents, distinguished guests or the rabbi. During the ceremony, the baby is seated on a special chair, called the "chair of Elijah", a symbolic representation of the prophet Elijah who, according to tradition, visits every celebration of the brit milah. The rite concludes with this blessing: "just as this child has now entered into the brit milah, may he merit to enter into the study of the Torah, to enter beneath the nuptial canopy and to perform good deeds!". Circumcision is in fact the only commandment in the Bible that is called a brit, meaning "covenant". Throughout Jewish history, it has become one of the distinctive symbols of the relationship between the Jewish people and the one God.
Source: Introduction to Judaism by Daniel Taub