Mekhilta d’Rabbi Yishmael: Bo -- Mesechta d’Piskha 5

“And you shall keep it...” Why did Scripture require that the paschal lamb (i.e. the Passover sacrifice) be taken four days before its sacrifice? R. Matia b. Heresh states: Behold, it says “Now when I passed by you, and looked upon you, and, behold, your time was the time of love (Ezekiel 16:8)” The time arrived for the Holy One to fulfill the promise made to Abraham that God would redeem his sons, but they had no mitzvot (commandments) to immerse themselves in order that they might be redeemed! As it says, “Your breasts were fashioned, and your hair was grown; you you were naked and bare;” that is, nude to all of the mitzvot. The Holy One gave them (the Israelites) two mitzvot: that of the paschal blood, and that of the circumcision blood, that they should busy themselves with them in order that they might be redeemed. As it says, “And when I passed by you, and saw you wallowing in your blood, I said unto you: In your blood, live; yea, I said unto you: In your blood, live...” And furthermore it says, “As for you also, because of the blood of your covenant I send forth your prisoners out of the pit wherein is no water.” (Zachariah 9:11) Therefore, Scripture required that the paschal lamb be taken four days before its sacrifice, for we are only rewarded with action. [Lauterbach translation. Edited for clarity and gender neutrality]