It was established by Pope Celestine V in 1294 with the issuing of the
Bull of Forgiveness. Pope Celestine V, born Pietro Angelerio del Morrone, was a hermit who in 1275 had temporarily found refuge in the Abbey of Santa Maria dell'Assunzione, on a promontory just outside the walls of L'Aquila called Colle di Maggio.
According to legend, it was here that he met the Virgin Mary in a dream and agreed with her to build a new majestic church in that place. On the death of Pope Nicholas IV on 4 April 1292, the Church found itself faced with a difficult papal election, in fact only two years later, on 5 July 1294, the conclave chose the new pontiff, the hermit Pietro del Morrone.
On 29 August of that year, in the Basilica of Santa Maria di Collemaggio, he was given the pontifical vestments and became pope with the name of Celestine V. On 29 September 1294 he promulgated the Bull of Forgiveness with which he granted a plenary indulgence to all humanity, setting only two conditions to obtain forgiveness:
entry into the Basilica of Santa Maria di Collemaggio – in the period of time between the evening of 28 and that of 29 August (the anniversary of his investiture) of each year – and being «truly repentant and confessed».

On 14 September In 1994, the Holy See, by mandate of Saint John Paul II, granted, in perpetuum, the Plenary Indulgence for August 29 to the Monastery of San Ruggero in Barletta. On the 30th anniversary of this privilege, the Monastic Community joins in prayer in the church of San Ruggero with the parish communities of the city for EUCHARISTIC ADORATION.