“Then Abraham circumcised his son Isaac when he was eight days old, as God had commanded him. Now Abraham was one hundred years old when his son Isaac was born to him.” Gen. 21:4-5
Isaac is circumcised. This was an important step for the son of promise, as we later see the anger of God toward Moses because he had not yet circumcised his son as commanded. (Ex. 4:24-26)
Some rabbis believe the eighth day signifies a new beginning (the cycle being based on the seven day creation week). Likewise in verse 5, we see Isaac’s birth coincides with Abraham’s centennial birthday. This represents a new chapter in Abraham’s life as well, not just in years, but now seeing the fulfillment of the son of promise.