“On the eighth day, the flesh of his foreskin shall be circumcised” (Leviticus 12:3) Another matter, “when a woman conceives” – that is what is written: “Distribute a portion to seven, and to eight as well” (Ecclesiastes 11:2). “Distribute a portion to seven,” these are the seven days of menstruation. “And to eight as well,” these are the eight days of circumcision. The Holy One blessed be He said: ‘If you observed the days of menstruation, I will grant you a son and you will circumcise him at eight days.’ That is, “on the eight day the flesh of his foreskin shall be circumcised.”
Why Circumcision Happens on the Eighth Day Not the First
Vayikra Rabbah 14:7