When Jesus instituted the Lord's Supper we are told that it was in the context of a Passover meal. Mark 14:12-18 says,

¹² On the first day of the Festival of Unleavened Bread, when it was customary to sacrifice the Passover lamb, Jesus’ disciples asked him, “Where do you want us to go and make preparations for you to eat the Passover?” ¹³ So he sent two of his disciples, telling them, “Go into the city, and a man carrying a jar of water will meet you. Follow him. ¹⁴ Say to the owner of the house he enters, ‘The Teacher asks: Where is my guest room, where I may eat the Passover with my disciples?’ ¹⁵ He will show you a large room upstairs, furnished and ready. Make preparations for us there.” ¹⁶ The disciples left, went into the city and found things just as Jesus had told them. So they prepared the Passover. ¹⁷ When evening came, Jesus arrived with the Twelve. ¹⁸ While they were reclining at the table eating, he said, “Truly I tell you, one of you will betray me—one who is eating with me.”

One detail of the meal that confirms this is found in Luke when the order of the meal found in Matthew and Mark is expanded in Luke to include another cup. Luke 22:17-23,

¹⁷ After taking the cup, he gave thanks and said, “Take this and divide it among you. ¹⁸ For I tell you I will not drink again from the fruit of the vine until the kingdom of God comes.” ¹⁹ And he took bread, gave thanks and broke it, and gave it to them, saying, “This is my body given for you; do this in remembrance of me.” ²⁰ In the same way, after the supper he took the cup, saying, “This cup is the new covenant in my blood, which is poured out for you. ²¹ But the hand of him who is going to betray me is with mine on the table. ²² The Son of Man will go as it has been decreed. But woe to that man who betrays him!” ²³ They began to question among themselves which of them it might be who would do this."

What you find here is Jesus instituting the Lord's Supper in the middle of a traditional Passover meal. The Passover meal had four cups of wine (which I am sure some people would like to carry over into the Lord's Supper today...just to follow the example, of course). So which two cups do we have represented in these verses? Let's look at another verse for more information, 1 Corinthians 10:16,

" Is not the cup of thanksgiving for which we give thanks a participation in the blood of Christ? And is not the bread that we break a participation in the body of Christ?" (NIV)

The NIV messes this one up a bit in calling it the "cup of thanksgiving." The word there is "eulogia" where we get the term eulogy or blessing. This is literally the cup of blessing (see NASB, NRSV). The cup of blessing is the cup Paul tells us was the cup Jesus used to institute the Lord's Supper. Where did the cup of blessing fit into the order of the 4 cups of the Jewish Passover? Here is their order:

1. Kiddush – Cup of   holiness
2. Maggid - Cup where the Passover story is told
3. Birkat Hamazon – Cup of blessing
4. Hallel – Cup of praise

The cup Jesus used for the Last Supper, to institute the Lord's Supper, was the third cup. So what about the fourth cup? Where is it? Jesus references that at the Last Supper,

"Truly I tell you, I will not drink again from the fruit of the vine until that day when I drink it new in the kingdom of God.”

Jesus is saying the fourth cup will not be partaken of until we all take it with him in the kingdom of God! We are truly living between the cups...between the cup of blessing that we partake of each week during the Supper and the Cup of praise that we will take with Christ in the kingdom!