Sometime after the creation of Adam and Eve on the sixth day of creation, see Genesis 1:26-31, they rebelled against God by eating the fruit of the tree of the knowledge of good and evil.
By doing so they brought sin into the world and upon all of humanity. The details of this are in Genesis 3:1-24.
“And if his offering is of the flocks – of the sheep or of the goats – as a burnt sacrifice, he shall bring a male without blemish. He shall kill it on the north side of the altar before the Lord; and the priests, Aaron’s sons, shall sprinkle its blood all around on the altar.” Leviticus 1:10-11.
