Holy Week is the week preceding Easter, from Passion (Palm) Sunday through Holy Saturday inclusive. During this week the Church commemorates the Passion of Christ, and all the ceremonies reflect this attitude of sorrow, yet joined with gratitude for God's mercy in becoming man in order to suffer and die for a sinful mankind.
The Triduum is period of three days of prayer, either preceding some special feast or preparing for some major enterprise. Commemorates the biblical three days that Christ lay in the tomb. Liturgically the Triduum is one liturgy consisting of Holy Thursday, Good Friday and the Easter Vigil.