Q - What is the Triduum?
A - Thanks for the question. The Triduum is made up of the three days before Easter - Holy Thursday, Good Friday and Holy Saturday. It is a single prayer of final preparation where we enter into the redemption of humanity and the salvation of the world made present in the Resurrection of our Lord.
This is the holiest part of the year and makes present the mystery of Jesus passion and death before He rises again.
Holy Thursday - The Mass of the Lord's Supper, it is the celebration of the first Eucharist in the upper room. This is when we have the annual washing of feet. Usually there is no other Mass celebrated on this day. Extra hosts are consecrated and then all of the Blessed Sacrament are taken from the Church and the tabernacle is left open to signify our longing for Christ. We have adoration after this mass as our last act of worship before the sorrow of Good Friday.
Good Friday - Celebration of the Lord's Passion. There is no Mass this day. Usually there are Stations of The Cross and a Communion service. This is when we have veneration of the Cross and the entire Passion of Christ is read by several lectors and the priest.
Easter Vigil - This is the high-point of the Church's year. During this celebration of Christ's death and Resurrection we have the RCIA candidates and elect receive the Sacraments of Initiation (Baptism, Confirmation, and Eucharist). The vigil must take place after night falls. It starts with an Easter fire outside of the Church. Then the paschal (Easter) candle is lit and processed into the Church. Then we all share the light of Christ with one another. Afterward, we have the Liturgy of the Word, which will have many readings about the story of God's Salvation history (7 Old Testament and 2 New Testament readings). Then after the homily, we celebrate baptism and confirmation. After this we celebrate the Eucharist. It is a long and absolutely beautiful liturgy with many "smells and bells".
We should prayerfully enter into the coming Holy Week in preparation for Christ's rising from the dead.
Christ have mercy on us all!