A Latin word meaning a space of three days, signifies in Catholic usage, a period of three consecutive days on which specified devotions are observed, determined prayers are said, or both, in order to obtain particular graces, to give thanks for special favors, to solemnize feasts, or to honor outstanding events as, for example, the election of a pope or the coronation of a king.
The choice of the number three for these devotions had its origin in a sacredness popularly attributed to it from pre–Christian times. In the OT, three–day periods were given particular importance (Tb 3.10; 6.16, 22; Jdt 12.6; Est 4.16; Dn 10.2–3; 2 Mc 13.12). In the NT, Our Lord referred to the three days Jonah spent in the whale's belly (Jn 2.1), and often spoke of the three days his own body would be in the tomb (Mt 17.22; 26.61; 27.40, 63; Mk 9.30).
Christians, very early, adopted the practice of a two– or three–day fast, in remembrance of our Lord's sojourn in the tomb, at different seasons of the year. In time, this led to the establishment of the liturgical observance of ember days, three days of fasting and special prayer, which were observed at the beginning of each of the four seasons. The days before Easter, namely, holy thursday, good friday, and Holy Saturday, came to be known as Triduum sanctum, the holy triduum, or more commonly, "Easter Triduum." In early medieval times, a three–day period of prayer and fasting came to be associated with all the important events of Catholic life. There was a triduum in preparation for Baptism, for the election of a pope, even for the end of the carnival.
Bibliography: l. duchesne, Origines du culte chrétien (5th ed. Paris 1925) 305–306. a. anwander, Wörterbuch der Religion (2d ed. Würzburg 1962) 111–112.
"Triduum." New Catholic Encyclopedia. . Encyclopedia.com. (January 19, 2019). https://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/triduum
"Triduum." New Catholic Encyclopedia. . Retrieved January 19, 2019 from Encyclopedia.com: https://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/triduum