Triduum comes from two Latin words—tres and dies—that means "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, spread over 72 hours.
The confusion is cleared up when we understand how the days are calculated. Christians count a day in the same way as Jews count days and festivals: 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 7 pm on Holy Thursday evening with the Mass of the Lord's Supper, continues with Good Friday, and concludes with our 7 pm Mass 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 through our baptisms in his death and resurrection. These days, then, are a reminder and celebration of who we ourselves are and what our own lives are about.
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