Triduum is a word from the Latin for “three days.” It refers to the celebrations that lead us from Lent into the Easter Season of Holy Thursday, Good Friday, Holy Saturday, and Easter Sunday. In the way we currently structure time, that is, from midnight to midnight, that means four days. But time used to be counted from sunrise to sunset. The Triduum begins at sundown on Holy Thursday and continues to sundown on Easter Sunday: three days. This time period celebrates the Paschal Mystery of Christ, not as a reenactment of Jesus’ life and death, but a time to ponder the meaning of that life, death, and resurrection in the past, in our present lives, and in the future to come.
General Norms for the Liturgical Year and Calendar, #18:
Christ redeemed humankind and gave perfect glory to God principally through his paschal mystery: dying he destroyed our death and rising he restored our life. Therefore the Easter triduum of the passion and resurrection of the Lord is the culmination of the entire liturgical year. Thus the solemnity of Easter has the same kind of preeminence in the liturgical year that Sunday has in the week.