According to wikipedia:
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.
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.
After the Gloria in Excelsis Deo at the Mass of the Lord's Supper all church bells are silenced and the organ is not used. so that the period that lasted from Thursday morning to before Easter Sunday began was once, in Anglo-Saxon times, referred to as "the still days". 
In the Roman Catholic Church, weddings, which were once prohibited throughout the entire season of Lent and during certain other periods as well, are prohibited during the Triduum. Lutherans still discourage weddings during the entirety of Holy Week and the Triduum.
Some comments from an Orthodox perspective:
1. These three days are not a distinct period of Holy Week, other than that the Bridegroom Services of Holy Week end on the evening of Holy Wednesday. All of Holy Week is a series which retraces the events prior to, and including, Christ's Passion, Crucifixion, burial and, on Easter Day itself, His Resurrection.
2. Orthodox weddings are expressly forbidden throughout the whole of Great Lent, as well as during Holy Week. Funerals are permitted during all this time, as are requiems during Great Lent. Funerals are permitted during Bright Week (the week between Easter and Thomas' Sunday), but not requiems. The form of a Bright Week funeral is, in essence, a condensed Resurrection Matins.
3. There is a beautiful and reverent Greek tradition for the Matins of Great Friday (the service held on Holy Thursday evening), where a single church bell is tolled at intervals during the chanting of Antiphon 15:
Today is hung upon a Tree, who hung the earth upon the waters. He is arrayed in a crown of thorns, who is King of the Angels. He is wrapped in the purple of mockery, who wraps the heavens in clouds. He receives blows on the face, who freed Adam in the Jordan. He is transfixed with nails, who is the Bridegroom of the Church. He is pierced by a lance, who is the Son of the Virgin. We venerate Your Passion, O Christ. We venerate Your Passion, O Christ. We venerate Your Passion, O Christ. Show us also Your glorious Resurrection.
4. A Slavic custom of Holy Thursday evening is the tolling of a single bell at the beginning of the reading of each Passion Gospel, with the number of tolls corresponding to the sequence of each Gospel. One for the first, two for the second, etc.