Triduum comes from two Latin words—tres and dies—that mean "a space of three days." But since we have four days with special names—Holy Thursday, Good Friday, Holy Saturday, and Easter Sunday—the "three" may be confusing to some. Nonetheless, they are all together one liturgical celebration.
The confusion is cleared up when we understand how the days are calculated. On all high festival days, the Church counts a day in the same way as Jews count days and festivals; that is, from sundown to sundown. Thus, the Triduum consists of three twenty-four-hour periods that stretch over four of our calendar days.
Therefore, the Easter Triduum begins at sundown on Holy Thursday with the Mass of the Lord's Supper, continues with Good Friday, and concludes with Easter evening prayer at sundown on Easter Sunday. Its high point is the celebration of the Easter Vigil.
During the Triduum we celebrate the core mystery of our Christian faith: we ritualize Jesus' transition from life to death to risen life, and our own participation in that timeless mystery.
It is so easy this time of year to celebrate these days as a historical commemoration. But we are doing far more than recalling historical facts. What Jesus did for us has consequences for all people for all time. His life, death, and resurrection happened to him, but they also happen to all of us who claim to be his followers. These days, then, are a reminder and celebration of who we ourselves are and what our own lives are about.
As we celebrate the mystery of Jesus' passing, we actually celebrate the same passing over in our own lives. Jesus' self-sacrifice opened the way for us to share in new life. But this does require our own cooperation in God's divine plan of salvation. We must pass over our lives into God's hands and imitate the self-giving of God's Son. This is the way to life. Our willingness to give of self for God's sake. "It is the Passover of the Lord."