Holy Thursday is the commemoration of the Last Supper of Jesus Christ, when he established the sacrament of Holy Communion prior to his arrest and crucifixion. Jesus celebrated the dinner as a Passover feast. Christ would fulfill His role as the Christian victim of the 
Passover for all to be saved by His final sacrifice.
The central observance of Holy Thursday is the ritual reenactment of the Last Supper at Mass. This event is celebrated at every Mass, as party of the Liturgy of the Eucharist, but it is specially commemorated on Holy Thursday.
He also establishes the special priesthood for his disciples, which is distinct from the “priesthood of all believers.” Christ washed the feet of his Disciples, who would become the first priests.
During the Passover meal, Jesus breaks bread and gives it to his Disciples, uttering the words, “This is my body, which is given for you.” Subsequently, he passes a cup filled with wine. He then says, “This is my blood…” It is believed those who eat of Christ’s flesh and blood shall have eternal life.
During the Mass, Catholics rightly believe, as an article of faith, that the unleavened bread and wine are transformed into the body and blood of Jesus Christ through a process known as transubstantiation. There have been notable Eucharistic miracles attributed to this event, such as bleeding hosts (communion wafers).
The Last Supper is celebrated daily in the Catholic Church as part of every Mass for it is through Christ’s sacrifice that we have been saved. On the night of Holy Thursday, Eucharistic Adoration of the Blessed Sacrament takes place where the faithful remain in the presence of the Eucharist just as the Disciples kept a vigil with Christ. Following the Last Supper, the disciples went with Jesus to the Mount of Olives, where he would be betrayed by Judas.
(Copied from Catholic.org)