It is an historical fact that at the establishment of the feast of Passover, God changed the Jewish calendar (Exodus 12). (034) This is important because Jesus is identified in 1 Corinthians 5:7 as "Christ, our Passover". Thus Jesus is the fulfillment of the significance of that Jewish feast.
To be the fulfillment, Jesus has to be the ultimate reality that the Passover pictured. And this change of the calendar, which is the first part of the picture which Jesus fulfilled, is heavy with implications because of the associations which Israel made with the months of their calendar.
It is a well known fact that Passover occurs in the spring. The instruction found in Exodus 12 says that Passover was to be observed on the fourteenth day of the first month. But the month designated by God as the first month for Israel was, originally, the seventh month of the calendar Israel was accustomed to.
The significance of this is found in the fact that the Egyptian world, in which the nation of Israel developed before the Exodus, associated the months with the constellations of the heavens (much like astrologers do today). And, their first month was called Tishri and corresponded to the constellation Libra. But, these constellations were extremely religiously significant. For the Egyptian world, the constellation Libra represented a religious doctrine: that acceptance before the gods depended upon human merit. If you were obedient to the varied wills of the gods, you would find acceptance in their eyes. If you were disobedient to them, they would relegate you to their equivalence of Hell. This was a gross distortion of Truth, and the God of Truth (Israel's God of History) wanted to erase that emphasis from the minds of His redeemed people. So, He shifted their calendar, and by so doing, shifted their focus of attention.
The calendar shift was from Tishri to Nisan. Nisan was the month in which the sun moved into the constellation Aries (the sacrificial Ram). It is no accident that God's Truth of His acceptance of people on the basis of the death of an Innocent Sacrifice had already been established for Israel by the sacrificial system that Abraham, Isaac, and Jacob had practiced. And Josephus says that Adam and Seth were the ones responsible for the dogma that was to be associated with the constellations. Thus, God shifted the focus of the New Year from a concept of acceptance by God through human good works, to a concept of acceptance by God through the death of an innocent substitute.
This is how Jesus is the ultimate reality of the earlier picture of Passover. In the Passover, the Jews were exempted from the judgment poured out on Egypt when the angel of death saw the blood of an innocent lamb (ram) on the door posts of the houses. Both the change of the calendar and the actual practice of deliverance by the blood of a sacrifice combined to change Israel's focus from an Egyptian dogma of salvation by human good works to a divine doctrine of salvation by the death of the Passover Lamb, Jesus Christ. Thus we know we have understood Jesus aright when we come to God on the basis of faith in Him rather than by faith in some good work we have done.