They put lamb's blood on their doors.
Lamb can be kosher for Passover. However, Ashkenazi Jews traditionally do not eat lamb during Passover.
It was to be a yearling male lamb or billy goat (Exodus 12). See also:More about Passover
The shank bone, typically displayed on the seder plate during Pesach or Passover, symbolized the sacrificial lamb. The blood from the lamb was supposedly used to paint the door and window frames of Jewish households to identify them to the angel of death when it slew the firstborn sons of the Egyptians, so that it would "pass over" those houses and spare the Jewish children.
John the Baptist calls Jesus the 'lamb of God'. - Joh 1:29. Here Jesus is being directly likened to the passover lamb. The passover lamb had to be perfect with out blemish. Jesus was born without sin, he lived a sinless life. The passover lamb had to die and it's blood was shed to save Israel's first born from death. In the same way it was necessary for Jesus to die as payment for sin. The lamb's blood had to be painted on the the door frames to show God the people were trusting Him for salvation. Salvation is the same today through faith in Jesus and His death as the payment for our sin.
This is because before the Exodus from Egypt when the 1st plague (water turning to blood) arrived, the Jews would put lamb's blood on their door so God would know to give them water (and not blood)
According to the book of Exodus in the Bible, the Israelites were instructed to sprinkle the blood of the Passover lamb on the doorposts and lintel of their homes as a sign for the angel of death to pass over them during the final plague in Egypt.
The Passover Lamb
It reminds them of the day when the destroying angel "passed over" their houses and looked for the blood of the unblemished lamb on their doorposts.
More specifically, it was the Passover lamb that is the type of Christ."Then Moses called for all the elders of Israel and said to them, 'Pick out and take lambs for yourselves according to your families, and kill the Passover lamb. And you shall take a bunch of hyssop, dip it in the blood that is in the basin, and strike the lintel and the two doorposts with the blood that is in the basin. And none of you shall go out of the door of his house until morning. for the Lord will pass through to strike the Egyptians and when He sees the blood on the lintel and on the two doorposts, the Lord will pass over the door and not allow the destroyer to come into your houses to strike you. And you shall observe this thing as an ordinance for you and your sons forever.'" Exodus 12:21-24Jesus became our Passover Lamb."Therefore purge out the old leaven, that you may be a new lump, since you truly are unleavened. For indeed Christ, our Passover, was sacrificed for us." 1Corinthians 5:7
Because he knew it was his last day on earth in his body with his disciples, and he gave it, his body and blood, for the disciples to remember him, and as a sign of the New Covenant in his blood and not the blood of a lamb, which is Passover.
A lamb.