Actually the reason it's done on the 8th day is because the blood doesn't clot before that. The babies will literally bleed out if done before.
My old pastor taught us this, and yes I believe him. His family was generational Satan worshipers in some high order, and through a series of crazy events he became a Christian. He now delivers others from all types of demon possessions. His entire family are believers as well. Their story, and their lives since, is simply amazing.