Triduum: the Three Days. These are our most holy of days. They make up our most important single celebration of the year: Easter. But exactly what three days make up the Triduum?
The three days of the Triduum are counted using the Jewish way of keeping time: from sunset to sunset. So the first day of the Triduum is from sunset on Holy Thursday until sunset on Good Friday. The second day is from sunset on Good Friday until sunset on Holy Saturday. The third day is from sunset Holy Saturday (the great Easter Vigil) to sunset Easter Sunday (Paschal Vespers).
We tend to think of the Three Days as commemorating separate, distinct events: on Holy Thursday we remember the Last Supper, on Good Friday we recall the passion and on Holy Saturday the resurrection. But in our liturgy, the church thinks about the Last Supper not as the last thing that happened on Holy Thursday, but as the thing to happen on Good Friday.
These Three Days are a single moment. We walk (or crawl) into this moment on Thursday night and walk (or dance) out on Easter day. In between, there’s a flood of stories and songs, rites and rest, fasting and feasting. The Three Days are time out of time, the center of our year and of our life.
(paraphrased from The Three Days, Gabe Huck)