Bnei Yisrael were commanded to designate a lamb on the 10th of Nissan to be slaughtered as the Pesach offering four days later.
Rashi comments that these four days of waiting were necessary because “the time for the Redemption had arrived... but Bnei Yisrael were steeped in idolatry.”
To leave Egypt’s borders but to bring its undesirable influences—“the shame of the earth”—with them would mean that the redemption was incomplete. The slaughtering of sheep—an Egyptian deity—for the Pesach sacrifice was the Jewish people’s means of rehabilitation. But the Jews in Egypt had not only dabbled in idolatry, they were steeped in it. Therefore, a one‑time act that renounced their previous obsession with idolatry was not enough to deprogram them from Egyptian influence. A longer process was required: four days of introspection.
Why four days?
When Avraham was commanded to offer his son Yitzchak as a sacrifice, he headed out the very next morning to do G‑d’s bidding, but he was shown the place where Yitzchak would be sacrificed only three days into his journey—i.e., four days after the initial command. Rashi explains that G‑d delayed showing it to him immediately, “so that people should not say that He confused him and confounded him suddenly, overwhelming his mind, and that if Avraham had had time to think it over, he would not have done it.”
A person’s greatest passion is his children. Giving Avraham four days to contemplate parting with his child meant that when he ultimately lifted the knife over Yitzchak, it was indisputable that he did so completely sound of mind.
Therefore, to remove the shame of the Egyptian influence, the Jewish people were commanded not only to slaughter a sheep, but to start the process four days in advance, fully aware of what they were about to do. By slaughtering a creature that they had once considered a deity, after four days of thought and awareness, they effectively and indisputably purged themselves of their idolatrous state of mind.
—Likkutei Sichos, vol. 16, pp. 117–119
