That is an excellent question and I believe there was a definite reason for that. Perhaps every believer in Christ knows that the “Passover Lamb” is a TYPE (or PICTURE) of the Lord Jesus Christ. The apostle Paul taught this truth in 1st Corinthians 5:7, “For indeed Christ, OUR PASSOVER, was sacrificed for us.” Years before that John the Baptist declared the truth that Christ was the Lamb of God, “The next day John saw Jesus coming toward him, and said, ‘Behold, the Lamb of God who takes away the sin of world” (John 1:29). And in 1st Peter 1:18-19 we have this wonderful truth: “Knowing that you were not redeemed with corruptible things, like silver and gold, from your aimless conduct received by tradition from your fathers, but with the precious blood of Christ, AS OF A LAMB WITHOUT SPOT OR BLEMISH.”

I would especially draw your attention to those last words, “as of a lamb without spot or blemish.” Christ had to be PERFECT (inwardly and outwardly) in order to be a sacrifice for sin that would satisfy God’s holiness. If He had not been perfectly sinless, He could not be a “substitute for sinners.” If He had sinned, or if He could sin, He would not have been HOLY, and He Himself would have needed a Savior. But He was perfect, and Scripture gives abundant evidence of this. In 1st Peter 2:22 we read, “Who DID no sin.” In 2nd Corinthians 5:21 we read, “Who KNEW no sin.” And in 1st John 3:5 we read, “In Him IS no sin.” This means, 1) He never “committed a sin” in thought, word or deed; 2) He never had any “experimental knowledge” of sin; and 3) He never had the “root of sin” in Him, which is a “sinful human nature” (see Romans 7:18-20). This is why the angel Gabriel told Mary, “That HOLY ONE who is to be born will be called the Son of God” (Luke 1:35). To be HOLY is to be “incapable of sin.”

Due to limited space, we won’t cite the many verses in the Old Testament that spoke of how the animal sacrifices that were offered for sin had to be “without spot or blemish.” This requirement was necessary in order for them to be “an acceptable sacrifice for sin” AND to be “a picture of Christ, the Lamb of God, who was without spot or blemish.” The Passover Lamb was no exception, for we read in Exodus 12:5, “Your lamb shall be WITHOUT BLEMISH, a male of the first year.” It was to be perfect with no flaws whatsoever (outwardly or hidden). Again, this was to be a perfect TYPE/PICTURE of Jesus Christ, the Lamb without spot or blemish. But then we read in verse 6, “Now you shall keep it until the fourteenth day of the same month. Then the whole assembly of the congregation of Israel shall kill it at twilight.” Why, as you have rightly asked, should they keep it until the 14th day before putting it to death? Ah, was it not to prove, over the course of those days, that it was indeed “a perfect lamb, without blemish?” All would be able to observe it and by careful scrutiny declare, “Yes, it is truly a perfect lamb and is suitable to be an acceptable sacrifice!” And so it was with the Lord Jesus Christ, the Lamb of God. He lived a “private life” for 30 years in Nazareth and a “public life” before all in Israel and during those 3 years He was being observed and tested by man, and before the congregation of Israel took Him by wicked hands and crucified Him (see Acts 2:23) it was proven that He was indeed “without spot or blemish.” Jesus challenged His enemies shortly before His death in John 8:46, “Which of you convicts Me of sin?” No one could step forward and point to any sin that He had ever committed! Why? Because He was indeed perfect in every way. He had said earlier in verse 29, “The Father has not left Me alone, for I always do those things that please Him.” No other human being could utter those words, for “all have sinned and come short of the glory of God” (Romans 3:23). But Jesus, the Lamb of God, could say them for every moment of every day He displayed His perfect holiness in everything He said and in everything He did.  (DO)  (520.3)