This is a special period of continuous prayer during which the Eucharist is exposed in a monstrance for adoration. The devotion begins with a Mass followed by continuous adoration over a 40-hour period, and it ends with a Mass and benediction. In the Bible the number 40 is associated with a sacred period of time: the rain at the time of the flood of Noah lasted 40 days and nights, and the Hebrews wandered in the desert for 40 years on the way to the Promised Land.
Jesus fasted for 40 days before beginning his public ministry. The devotion was promoted by both Saint Philip Neri and Saint Ignatius of Loyola in the 1500s. In the United States, Saint John Neumann (1811-1860), bishop of Philadelphia, helped spread the devotion.
The Importance of Eucharistic Devotions
Although the Mass is central to Catholic life, devotions to the Eucharist are also important. Through them we can adore Jesus, pray in his presence, express our sorrow for sin, and experience a oneness with him that leads us to participate in the Mass.
source: loyolapress
