From the USCCB: http://www.usccb.org/prayer-and-worship/liturgical-year/triduum/
The summit of the Liturgical Year is the Easter Triduum—from the evening of Holy Thursday to the evening of Easter Sunday. Though chronologically three days, they are liturgically one day unfolding for us the unity of Christ’s Paschal Mystery.
The single celebration of the Triduum marks the end of the Lenten season, and leads to the Mass of the Resurrection of the Lord at the Easter Vigil.
The liturgical services that take place during the Triduum are:
- Mass of the Lord’s Supper
- Good Friday of the Lord’s Passion
- Mass of the Resurrection of the Lord