This evening — Thursday of Holy Week — marks the beginning of the GREAT TRIDUUM. The liturgical Triduum includes three services: Holy Thursday, Good Friday and the Easter Vigil (Saturday). In many traditions, this is actually considered one continuous worship service, starting Thursday evening and ending at the vigil.
Historically, on Holy Thursday we focus on and reflect upon the Lord’s Supper. This last supper was a Passover Seder and Jesus appropriates the symbols of the Pesach to explain what was about to happen to him and the significance of what was to come — that which we pass on, generation to generation.
As the Apostle Paul would write years after the historical events of the crucifixion and resurrection of Jesus:
I Corinthians 11:23-26
23For I received from the Lord what I also passed on to you: The Lord Jesus, on the night he was betrayed, took bread, 24and when he had given thanks, he broke it and said, “This is my body, which is for you; do this in remembrance of me.” 25In the same way, after supper he took the cup, saying, “This cup is the new covenant in my blood; do this, whenever you drink it, in remembrance of me.”26For whenever you eat this bread and drink this cup, you proclaim the Lord’s death until he comes.