[2] The practice of reserving the Blessed Sacrament with some solemnity during the Easter Triduum began in the 12th or 13th century, From this the idea grew up of transferring this figurative vigil of forty hours to other days and other seasons.
In November 2021, the Roman Catholic Diocese of Arlington initiated a schedule of Forty hours Devotion in successive parishes in preparation for its Golden Jubilee in 2024.
[5] A Dominican, Father Thomas Nieto, the Barnabite St. Antonio Maria Zaccharia, and his friend Brother Buono of Cremona, known as the Hermit, have all been suggested as the founders of the Forty Hours' Devotion.
But the evidence in all these cases only goes to show that the practice was then being introduced of exposing the Blessed Sacrament with solemnity on occasions of great public calamity or peril, and that for such expositions the period of forty hours was generally selected.
The faithful would come in shifts before the Sacrament seeking God's intercession during events threatening the local community, such as war, epidemics, drought or famine.
One of the most important documents pertaining to this devotion is the Constitution Graves et diuturnae of Pope Clement VIII, of 25 November 1592, in which the pontiff strongly commended the practice of unwearied prayer.We have determined to establish publicly in this Mother City of Rome (in hac alma Urbe) an uninterrupted course of prayer in such wise that in the different churches (he specifies the various categories), on appointed days, there be observed the pious and salutary devotion of the Forty Hours, with such an arrangement of churches and times that, at every hour of the day and night, the whole year round, the incense of prayer shall ascend without intermission before the face of the Lord.
Before the year 1550 this, or some analogous exposition, had been established by St. Philip Neri for the Confraternity of the Santissima Trinità dei Pellegrini in Rome; while St. Ignatius Loyola, also encouraged the practice of exposing the Blessed Sacrament during the carnival as an act of expiation for the sins committed at that season.
[4] St. Francis de Sales, who was a great admirer of Philip Neri, incorporated the Forty Hours Devotion as part of his missionary outreach among the Calvinists living between Annecy and Geneva between 1594 and 1596.
In the United States, Saint John Neumann (1811-1860), bishop of Philadelphia, helped spread the devotion, composing a special booklet for its practice.