27 Wherefore, confiding in the mercy of Almighty God and in the authority of the Blessed Apostles Peter and Paul, by virtue of that power of binding and loosing which, unworthy though We are, the Lord has given Us, We do concede and impart the most plenary indulgence of all their sins to the faithful, all and several of both sexes, dwelling in this Our beloved City, or coming into it, who from the first Sunday in Lent, that is from the 21st of February, to the second day of June, the solemnity of the Most Sacred Body of Christ, inclusively, shall three times visit one of the four Patriarchal basilicas, and there for some time pray God for the liberty and exaltation of the Catholic Church and this Apostolic See, for the extirpation of heresies and the conversion of all who are in error, for the concord of Christian Princes and the peace and unity of all the faithful, and according to Our intention; and who, within the said period, shall fast once, using only meager fare, excepting the days not included in the Lenten Indult; and, after confessing their sins, shall receive the most holy Sacrament of the Eucharist; and to all others, wherever they be, dwelling outside this city, who, within the time above mentioned or during a space of three months, even not continuous, to be definitely appointed by the ordinaries according to the convenience of the faithful, but before the eighth day of December, shall three times visit the cathedral church, if there be one, or, if not, the parish church; or, in the absence of this, the principal church; and shall devoutly fulfill the other works abovementioned. And We do at the same time permit that this indulgence, which is to be gained only once, may be applied in suffrage for the souls which have passed from this life united in charity with God.
Source: Ad Diem Illum Laetissimum (Vatican.va)