1) Latin religious phrase 
Latin, meaning: a period of three days 
A triduum is a religious observance lasting three days. The best-known example today is the liturgical Paschal Triduum (the three days from the evening of Maundy Thursday to Easter Sunday). Other liturgical tridua celebrated in Western Christianity include Hallowtide, the Rogation Days, and the feasts of Easter and Pentecost together with the firs...
period of three days of prayer preceding Roman Catholic feasts
In The Roman Catholic religion, a triduum is three days of prayer in preparation for a feast or other solemn occasion.
the three days of the liturgical year which incorporate the celebrations of Holy Thursday, Good Friday and Holy Saturday's Easter Vigil. It ends with Evening Prayer on Easter Day.
a period of three days.