Triduum - Holy Week

What does that mean?

Easter Triduum, Holy Triduum, or Paschal Triduum is the period of three days from Holy Thursday (seen as beginning with the service of the preceding evening) to Easter Day. It begins with the Thursday Mass of the Lord's Supper and ends with evening prayer on Sunday.[1]

Since the 1955 reform by Pope Pius XII, the Easter Triduum, including as it does Easter Sunday, has been more clearly distinguished as a separate liturgical period. Previously, all these celebrations were advanced by more than twelve hours. The Mass of the Lord's Supper and the Easter Vigil were celebrated in the morning of Thursday and Saturday respectively, and Holy Week and Lent were seen as ending only on the approach of Easter Sunday.

And now you know!
Wikipedia