In Numbers 9:1-5, we're told:
The Lord spoke to Moses in the wilderness of Sinai, in the first month of the second year after they had come out of the land of Egypt, saying: 2Let the Israelites keep the passover at its appointed time. 3On the fourteenth day of this month, at twilight, you shall keep it at its appointed time; according to all its statutes and all its regulations you shall keep it. 4So Moses told the Israelites that they should keep the passover. 5They kept the passover in the first month, on the fourteenth day of the month, at twilight, in the wilderness of Sinai. Just as the Lord had commanded Moses, so the Israelites did.Passover begins on the 14th day of Nisan. Nisan is the first month of the Jewish calendar. By the reckoning of this calendar, new days begin at sundown. Today is the 14th of Nisan, 5771.
Passover, of course, remembers the events recorded in Exodus 12. There, we're told of what happened when God sent a tenth and final plague to Egypt designed to gain the freedom of His chosen people--the Hebrews or the Israelites--from slavery. They had been slaves in Egypt for 430 years. The tenth plague came when God sent the angel of death to bring the deaths of all the first-born in Egypt.
The Israelites were spared though because, as God instructed them through Moses, they had previously smeared the blood of unblemished lambs on the doorposts of their dwellings. The lambs were offered to God in place of the firstborn Israelites. When the angel of death encountered the blood of the lambs, it passed over, ensuring that the first-born in those dwellings didn't die.
Blood, of course, carries life. In the Bible's account of the first murder, when Cain killed his brother Abel, God said that the blood of Abel cried out.
On the Day of Atonement, Yom Kippur on the Jewish calendar, in Temple times, an unblemished lamb was offered as sacrifice signaling repentance and bringing reconciliation (atonement) with God, on behalf of the people. Its blood covered the sins the people committed fin the preceding year. As part of the rite on this day, the priest sprinkled the blood of the sacrificed lamb on the people. The blood of the lamb, in effect, called out to God and God passed over their sins. The significance of this can be seen when we consider the statement of a devout Jew who came to follow Jesus as Messiah, Lord, and God, the apostle Paul writing in Romans 6:23, "The wages of sin is death." By God's gracious intervention, the ancient Israelites were spared the proper "wages" for sin; God allowed the unblemished lamb to take the place of His people.
We Christians believe that just as the blood of unblemished lambs on the doorposts of the Hebrew dwellings in Egypt caused the angel of death to pass over God's people, leading ultimately to the Hebrews' liberation from slavery, the blood of Christ, covering the sins of repentant believers in Him, brings freedom from sin and death.
Jesus is, as John the Baptizer put it, "the Lamb of God, Who takes away the sin of the world." And this sacrifice needn't be made repeatedly, as was true of the lambs of Yom Kippur. Jesus' sacrifice is effective once and for all. As the New Testament book of Hebrews puts it, Christ, the highest of priests, sacrificed Himself, so that all who repent and believe in Him are forgiven their sins, ever reconciled to God, and given everlasting life with God.
Paul puts the connection between the Jewish Passover and the Christian commemorations of Good Friday and Easter succinctly when he says:
Christ, our Passover lamb, has been sacrificed...(1 Corinthians 5:7, TNIV)[Please also read Ephesians 2:11-13. Without faith in Jesus Christ, we stand outside a relationship with God. But we are brought near "by the blood of Christ."]