"At the age of eight days every male child shall be circumcised"
A healthy male child is circumcised at eight days old, even if that day is
Shabbos. This mitzvah was the first one given to Avraham Avinu. It is so
great that it is called "bris," a covenant, thirteen times. (There are
two other mitzvos that are also called "bris" - Shabbos and Torah.)
There are only two positive commands which carry the penalty of kareis (a severe
punishment of being spiritually "cut off")if not fulfilled: bris milah and
korban Pesach. Bris Milah is the individual's physical bond to Hashem,
while the korban Pesach is similar communal attachment of the nation to
Bris milah is so great that it is equal to all of the other mitzvos in the
Torah (Nedarim 32a).