The resurrection of Jesus from the dead is the heart of the Good News about Jesus. The Easter triduum, which marks the end of Lent, begins at the evening Mass of the Lord's Supper and spans three days – Good Friday, Holy Saturday and Easter Sunday.
Although it spans three days, it is one event. The triduum is not really three liturgies, it is one long liturgy. One celebrates the three days not just by attending Holy Thursday's celebration of the Lord's Supper, or by attending Good Friday's liturgy (which is not a Mass), or by simply going to the Easter Vigil liturgy. No, one celebrates the triduum by attending all three of those services. It's all one liturgy!
Everything in the Old Testament flows toward these three days, and everything in the New Testament flows from them. They are the core of all that is Christian.
These three great days are grounded in the paschal mystery. "Paschal" comes from the Jewish word Pesach, the "passing over,". God is faithful to his covenant and, as Christians, we believe he has fulfilled his promises. Christ, the mystical lamb, joined us into himself and brings us with him in his triumph over death into eternal life.
Having over these three days entered into Christ's Passover, we are sent forth to bring its power into our world. Water (baptism) and blood (the Eucharist) flowed from Christ's pierced side when he was sacrificed on the altar of the cross. His life flows out into us now in his Church's sacraments. God "passes over" our sins because we are justified in Christ's merciful and sacrificial "Passover." Truly, we are saved by the blood of the Lamb of God.
The triduum is the core of everything that is Christian. Every celebration of the Mass is a recapitulation of all that Christ accomplished in the paschal mystery, which is why we refer to it as the holy sacrifice of the Mass.
Fr. Antonius David Tristianto, O.Carm.