It appears from the gospels that the evening meal known as The Last Supper takes place on the first day of Passover, which would make this the Seder meal. Right after the disciples find the upper room, these gospels tell us that when evening came, the Lord Jesus had the meal there with the disciples. This meal would include lamb, bitter herbs, and unleavened bread. These three items were accompanied by other foods and there would be four cups of wine. A person would celebrate this meal with their family and we see Jesus celebrating it with His closest family: the disciples. My husband and I have eaten several Seder meals with my mother-in-law at her church and I found it to be very meaningful and spiritual to take part in a dinner our Lord also took part in. The Passover celebrates the rescue of Israel from their slavery in Egypt and the passing over of the tenth plague which killed all the firstborn of Egypt, for the Israelites sacrificed lambs and painted the blood on the doorposts, and then the angel of death passed over their houses. Today, as Christians, we can celebrate Passover because of the new covenant made between God and man with the blood of Christ. We celebrate our rescue from the slavery of sin and our assurance that we will be passed over and saved from eternal separation from God by the blood of the Lamb: the Lord Jesus Christ.
The third cup is poured but not consumed until after the meal has been eaten. This blessing would be said prior to the meal, "Blessed are You, Lord God, who brings forth bread from the earth." Then, after the meal, a blessing would be said over the third cup and they would drink it. Then the fourth cup is poured.
Before the fourth cup they would recite the Hallel, which is comprised of Psalms 113-118, which you may want to read to see the exact Psalms that Jesus and the disciples recite at this point. It is believed that this fourth cup is the one that Jesus does not drink of but says, "I tell you, I will not drink from this fruit of the vine from now on until that day when I drink it new with you in My Father's kingdom." (Matthew 26:29) This symbolizes what would be said at the end of a meal, "Next year in Jerusalem!" I had a difficult time trying to interpret the meaning of this. In one sense it represents the Jews who are dispersed thru the world (the Diaspora, a word which means "to scatter seed", indicating the Jews being scattered among the nations) and a looking toward Jerusalem as the holy city of God and their inability to be there to celebrate the Passover feast. It also hearkens back to the times when Israel was captive in other lands and longed to be in Jerusalem. In another sense it's the looking forward to the coming of Messiah and His kingdom. It signifies the hope of Israel that Messiah will come before Passover of the next year. Jesus, in saying that He wont drink of the cup until He drinks it new in the kingdom, is saying that He will share this cup in the New Jerusalem with those who have been redeemed by His blood. We are still waiting to drink the fourth cup with Him in the Father's kingdom. Jesus has reserved that celebratory cup to drink with us there. When Jesus concluded the meal by saying, "Next year in Jerusalem!", He meant the New Jerusalem, not the earthly city. When followers of Christ celebrate the Seder meal, we are looking forward to that new city made by ours Lord.
Luke tells us that, "When the hour came, Jesus and His disciples reclined at the table." (Luke 22:14) This is how we know it's the fourth cup after the meal that Jesus doesn't drink of because they are reclining now. The Passover meal was meant to be eaten standing up, just as the children of Israel ate it standing up before the Exodus from Egypt. They have finished the meal and are reclining at the table now. Jesus presents to the disciples the new covenant of His blood at this Passovee. He uses this final Passover with the disciples to present Himself as the Lamb who takes away the sins of the world. These cups that are consumed during the Passover with His disciples represent two characteristics of God: mercy and judgment. God mercifully rescued Israel from Egypt and mercifully passed over their firstborn sons by passing over those homes that had the blood of the lamb applied to them. At the same time, as a righteous God, He has to judge sin and so we owe a debt for our transgressions. A lamb was used sacrificially on a yearly basis at Passover not just as an observance of rescue from Egypt but as an observance that we are sinners and that no redemption is accomplished without bloodshed. The lambs of the sacrificial system were a substitution for the blood the sinner owed for his sins. Rather than taking our lives as the penalty, God accepted our repentance and the blood of sacrifices in place of our own blood.
God, in His great mercy, spared the firstborn of Israel at the first Passover. But also in His great mercy He didn't spare His own firstborn Son. God gave His one and only Son, perfect and holy and without any sin, so that we sinners could become the sons and daughters of God. We are joint-heirs with Christ in the kingdom of our God. (Romans 8:17) Christ our Passover is sacrificed for us (1 Corinthians 5:7). The Lamb of God who takes away the sins of the world (John 1:29) willingly laid down His life for us and shed His blood for us on a cross like a criminal. We, the true criminals, get into the kingdom of God on grace alone. We will someday bow before the One who took our sins upon Himself. We will behold the nail scars in His hands and feet. We will see the scar where the spear entered His heart which was broken for us. And we will praise the one and only holy spotless Lamb of God forever and ever. We bless Your name, Lord Jesus, that You, the sinless One, gave Your life for us. We were sinners, broken and useless and unprofitable in every way, yet You loved us and allowed Your body to be broken and Your blood to be spilled on our behalf. We glorify and praise Your name for this indescribable gift. Amen.