Thursday of Holy Week is usually called "Holy Thursday" and sometimes called "Maundy Thursday". Maundy comes from the Latin for for mandate and is used to recall the mandate Jesus gives his followers at the Last Supper to "do this in remembrance of me." Personally, I don't like the mandate language and, since there was much more to Thursday of Holy Week than the Last Supper I prefer to older "Holy Thursday" title.
Holy Thursday is a busy day. It begins with Jesus sending two of his followers into Jerusalem to make preparations for the Seder (the Jewish ceremonial Passover meal). The Seder recalls and remembers the Exodus experience of the children of Israel as they are delivered from Pharaoh by the plagues that culminate in the angel of death. This angel "passes over" the Israelites (hence the title Passover).
It is at this meal that Jesus takes the the broken unleavened bread and says "this is my body broken for you". It is at this meal that Jesus takes the cup of blessing and offers it to his friends and says "this is my blood poured out for you."
But here is so much more going on. The betrayer is at table with Jesus. There is conversation about "who's the greatest?" there is conversation about everyone denying Jesus when times get tough.
Then there is the Garden of Gethsemane. Jesus prays for help, but submits to his Father's will. Then comes Judas and the soldiers and Jesus is arrested and taken to the religious authorities for trial. They determine that Jesus should die and that he should die at the hands of the Romans (by crucifixion rather than stoning). Peter denies Jesus three times before the rooster crows. This is all part of a long dark night leading into Good Friday.