A celebration of such a devotion is begun by a Solemn Mass or "Mass of Exposition", and ended by a "Mass of Deposition". Each of these masses includes the Blessed Sacrament being processed and the litanies of the saints being chanted.
The exact period of forty hours' exposition is not in practice very strictly adhered to; for the Mass of Deposition is generally sung at about the same hour of the morning, two days after the Mass of Exposition. On the intervening day a solemn Mass pro pace is offered -- if possible, at a different altar from the high altar upon which the Blessed Sacrament is exposed. It is assumed that the exposition and prayer should be kept up by night as well as by day, but permission is given to dispense with this requirement when enough watchers cannot be obtained. In such a case the interruption of the devotion by night does not forfeit the indulgences conceded by the Holy See to those who take part in it.
The parchment is endorsed on the back in a contemporary hand, "The first concession of Indulgence" etc., and we may feel sure that this is the earliest pronouncement of the Holy See upon the subject. But the practice spread rapidly, though the details cannot be traced exactly. Already before the year 1550 this, or some analogous exposition, had been established by St. Philip Neri for the Confraternity of the Trinita dei Pellegrini in Rome; while St. Ignatius Loyola, at encouraged to the practice of exposing the Blessed Sacrament during the carnival as an act of expiation for the sins committed at that season. As this devotion also commonly lasted for a period of about two days or forty hours, it seems likewise to have shared the name "Quarant' Ore"; and under this name it is still maintained in many places abroad, more especially in France and Italy. This practice was especially promoted by the Oratorian Father, Blessed Juvenal Ancina, Bishop of Saluzzo, who has left elaborate instructions for the carrying out the devotion with greater solemnity and decorum. It seems that it is especially in connection with these exercises, as they flourished under the direction of the Oratorian Fathers, that we trace the beginning of those sacred concerts of which the memory is perpetuated in the musical "Oratorios" of our greatest composers. Elaborate instructions for the regulation of the Quarant' Ore and for an analogous devotion called "Oratio sine intermissione" (uninterrupted prayer) were also issued by St. Charles Borromeo and will be found among the Acta Mediolanensis Ecclesiae". However, the most important document belonging to this matter is the Constitution "Graves et diuturnae" of Pope Clement VIII, 25 November 1592. In the presence of numberless dangers threatening the peace of Christendom and especially of the distracted state of France, 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". Notice that, as in the case of the previously cited Brief of Paul III, the keynote of this document is anxiety for the peace of Christendom. "Pray for the concord of Christian princes, pray for France, pray that the enemies of our faith the dreaded Turks, who in the heat of their presumptuous fury threaten slavery and devastation to all Christendom, may be overthrown by the right hand of the Almighty God". Curiously enough the document contains no explicit mention of the exposition of the Blessed Sacrament, but inasmuch as this feature had been familiar on such occasions of public prayer both in Milan and at Rome itself for more than half a century, we may infer that when the pope speaks of "the pious and salutary devotion of the Forty Hours" he assumes that the prayer is made before the Blessed Sacrament exposed. In 1731 Pope Clement XII issued a very minute code of instructions for the proper carrying out of the Quarant' Ore devotion, which is known as the "Instructio Clementina".
With regard to the actual originator of the Forty Hours' Devotion there has been much difference of opinion. The evidence seems to favour the conclusion that a Capuchin Father, Joseph Piantanida da Fermo, was the first to organise the arrangement by which the Forty Hours' Exposition was transferred from church to church in Milan and was there kept up without interruption throughout all the year (see Norbert in the" Katholik", August 1898). On the other hand, the practice of exposing the Blessed Sacrament with solemnity for forty hours was certainly older; and in Milan itself there is good evidence that one Antonio Bellotto organized this in connexion with a certain confraternity at the church of the Holy Sepulchre as early as 1527. Moreover, 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. The claims of the last named, Brother Buono, were urged by Bergamaschi ("La Scuola Cattolica", Milan, September 1908, 327-333), who contends that the Quarant' Ore had been started by Brother Buono at Cremona in 1529. 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. That this period of forty hours was so selected seems in all probability because this was about the length of time that the Body of Christ remained in the tomb, and that the Blessed Sacrament in the Middle Ages was left in the Easter Sepulchre. St. Charles Borromeo speaks as if this practice of praying for forty hours was very ancient; and he distinctly refers it to the forty hours that Christ's Body remained in the tomb, seeing that this was a period of watching, suspense and ardent prayer for all His disciples. In all probability this was the exact truth. The practice of reserving the Blessed Sacrament with some solemnity in the Easter Sepulchre began in the thirteenth or fourteenth century; and seems in some places, e.g. at Zara in Dalmatia, to have been popularly known as the "Prayer [or Supplication] of the Forty Hours". From this the idea grew up of transferring this figurative vigil of forty hours to other days and other seasons. The transference to the carnival tide was very obvious, and is likely enough to have occurred independently to many different people. This seems to have been the case with Father Manare, S.J., at Macerata, c. 1548, but probably the idea suggested itself to others earlier than this.