1)
Why is the Bris Milah performed on the eighth day?
1.
Maharal (Tif'eres Yisrael Ch. 2, p. 10): The natural world was created in seven days, and a person in his original natural state was created with the deficiency of an Orlah. The Milah, a rectification of nature, is therefore done on the eighth day. 1
1
Refer to 17:9:1.1:1.