The redemption of B’nei Yisrael (the Children of Israel) began on Shabbos, specifically, the tenth of Nissan. That Shabbos, later, became known as Shabbat HaGadol. What was so special about the Tenth of Nissan? That day was when the Children of Israel were commanded to bring a lamb into each and every one of their homes. “Speak ye unto all the congregation of Israel, saying: In the tenth day of this month they shall take to them every man a lamb, according to their fathers’ houses, a lamb for a household” (Exodus 12:3, JPS 1917 Tanach).
Why was this the beginning of the Redemption for B’nei Yisrael? Because the lamb was to be the first national offering, made by each and every family, for the sake of using the blood of the lamb as a sign, placed upon the doorposts and lintels of their homes. The blood would serve as a sign, whereby, “the L-RD will pass through to smite the Egyptians; and when He seeth the blood upon the lintel, and on the two side-posts, the L-RD will pass over the door, and will not suffer the destroyer to come in unto your houses to smite you” (Exodus 12:23).
And, so, the tenth plague, the slaying of the first born, was not enacted upon the Children of Israel. They were spared, because of their emunah (faith) in H’Shem, that compelled them to carry out the commandment, regarding the Pesach lamb. They had been further commanded, “none of you shall go out of the door of his house until the morning” (Exodus 12:23).
“Come, my people, enter thou into thy chambers, and shut thy doors about thee; hide thyself for a little moment, until the indignation be overpast.”
– Isaiah 26:20, JPS 1917 Tanach