Holy Week is referred to as such on account of the three days that cap off the week and their significance in Christian understanding. These three days, which directly follow what is commonly called Spy Wednesday, represent the Paschal Triduum. During this concise season within the Catholic Church’s liturgical calendar, the faithful observe “the most exalted liturgical celebration of the year.”
19. The Easter triduum begins with the evening Mass of the Lord's Supper, reaches its high point in the Easter Vigil, and closes with evening prayer on Easter Sunday. (General Instruction on. the Liturgical Calendar). The day of Holy Thursday is NOT part of the Triduum. The Evening Mass of the Lord's Supper begins the tritium just like the Saturday Evening Vigil Masses begin the Sunday Celebration. The Paschal Triduum if Good Friday, Holy Saturday and Easter Sunday. This is clear in the Instruction. Before writing it would be good for people to read the sources.