I'm just going to throw this out there and feel free to correct me if you think I'm wrong.
I believe they were having the passover meal that night. During the passover meal they go through the story of the exodus from Egypt and the bitterness of their slavery and all the wonders that God performed to free them. They killed the passover lamb and put the blood on the door post. This was the shadow of things to come.
I believe that when Jesus said do this in remembrance of me he was saying he is the passover lamb and the spiritual fulfillment of the passover. They should look to him now as the sin offering and freedom from slavery. The new covenant. I know I haven't worded that quite right.
Luke 22
14And when the hour was come, he sat down, and the twelve apostles with him.
15And he said unto them, With desire I have desired to eat this passover with you before I suffer:
16For I say unto you, I will not any more eat thereof, until it be fulfilled in the kingdom of God.
17And he took the cup, and gave thanks, and said, Take this, and divide it among yourselves:
18For I say unto you, I will not drink of the fruit of the vine, until the kingdom of God shall come.
19And he took bread, and gave thanks, and brake it, and gave unto them, saying, This is my body which is given for you: this do in remembrance of me.
20Likewise also the cup after supper, saying, This cup is the new testament in my blood, which is shed for you.
Hebrews 10
1For the law having a shadow of good things to come, and not the very image of the things, can never with those sacrifices which they offered year by year continually make the comers thereunto perfect.
2For then would they not have ceased to be offered? because that the worshippers once purged should have had no more conscience of sins.
3But in those sacrifices there is a remembrance again made of sins every year.
4For it is not possible that the blood of bulls and of goats should take away sins.
5Wherefore when he cometh into the world, he saith, Sacrifice and offering thou wouldest not, but a body hast thou prepared me:
6In burnt offerings and sacrifices for sin thou hast had no pleasure.
7Then said I, Lo, I come (in the volume of the book it is written of me,) to do thy will, O God.
8Above when he said, Sacrifice and offering and burnt offerings and offering for sin thou wouldest not, neither hadst pleasure therein; which are offered by the law;
9Then said he, Lo, I come to do thy will, O God. He taketh away the first, that he may establish the second.
10By the which will we are sanctified through the offering of the body of Jesus Christ once for all.
11And every priest standeth daily ministering and offering oftentimes the same sacrifices, which can never take away sins:
12But this man, after he had offered one sacrifice for sins for ever, sat down on the right hand of God;
13From henceforth expecting till his enemies be made his footstool.
14For by one offering he hath perfected for ever them that are sanctified.
15Whereof the Holy Ghost also is a witness to us: for after that he had said before,
16This is the covenant that I will make with them after those days, saith the Lord, I will put my laws into their hearts, and in their minds will I write them;
17And their sins and iniquities will I remember no more.
18Now where remission of these is, there is no more offering for sin.
19Having therefore, brethren, boldness to enter into the holiest by the blood of Jesus,
20By a new and living way, which he hath consecrated for us, through the veil, that is to say, his flesh;
21And having an high priest over the house of God;
22Let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience, and our bodies washed with pure water.
23Let us hold fast the profession of our faith without wavering; (for he is faithful that promised;)