It is finished refers to the live passover in which Jesus was the sacrificial Lamb. If you read about the Passover meal, there are 4 cups of wine drunk at Passover. After the fourth cup, the blessing that concludes the Seder meal and it is over. Jesus was given the 4th cup while on the cross it was after he drank from the reed that he said this. It is the end of the OLD covenant and at Passover the night before, we had the beginning of the NEW covenant to be members of the Church that Christ established, when he said in Matthew 26:26 And whilst they were at supper, Jesus took bread, and blessed, and broke: and gave to his disciples, and said: Take ye, and eat. This is my body. 27 And taking the chalice, he gave thanks, and gave to them, saying: Drink ye all of this. 28 For this is my blood of the new testament(covenant), which shall be shed for many unto remission of sins. 29 And I say to you, I will not drink from henceforth of this fruit of the vine, until that day when I shall drink it with you new in the kingdom of my Father. 30 And a hymn being said, they went out unto mount Olivet. This was solidified on Pentecost when the Holy Spirit came to dwell amongst us and lead us in establishing the Church that Christ founded.