This is a solemn commemoration, in certain Dioceses and congregations, of the Saints whose Relics are preserved in their churches. From the first days of Christianity, Mass was celebrated by preference on the burial-place of the Martyrs. Nowadays a square flat stone is placed in the middle of the altar, and this altar-stone contains one or more relics of Saints.
It has long been customary, especially in churches which possessed large collections of relics, to keep one general feast in commemoration of all the saints whose memorials are there preserved. An Office and Mass for this purpose will be found in the Roman Missal and Breviary; and though they occur only in the supplement Pro aliquibus locis (Masses for Certain Places) and are not obligatory upon the Church at large, still this celebration is now kept almost universally. The office is generally assigned to the fourth Sunday in October. In England before the Reformation, as we may learn from a rubric in the Sarum Breviary, the Festum Reliquiarum was celebrated on the Sunday after the feast of the Translation of St. Thomas of Canterbury (7 July), and it was to be kept as a greater double “wherever relics are preserved or where the bodies of dead persons are buried, for although Holy Church and her ministers observe no solemnities in their honour, the glory they enjoy with God is known to Him alone.”
Responding to concerns expressed by the protestants, the 25th Session of the Council of Trent had this to say about the veneration of relics (emphases and scriptural references added):
Moreover, that the images of Christ, of the Virgin Mother of God, and of the other saints, are to be had and retained particularly in temples, and that due honour and veneration are to be given them; not that any divinity, or virtue, is believed to be in them, on account of which they are to be worshipped; or that anything is to be asked of them; or, that trust is to be reposed in images, as was of old done by the Gentiles who placed their hope in idols; but because the honour which is shown them is referred to the prototypes which those images represent; in such wise that by the images which we kiss, and before which we uncover the head, and prostrate ourselves, we adore Christ; and we venerate the saints, whose similitude they bear: as, by the decrees of Councils, and especially of the second Synod of Nicaea, has been defined against the opponents of images.
And the bishops shall carefully teach this,-that, by means of the histories of the mysteries of our Redemption, portrayed by paintings or other representations, the people is instructed, and confirmed in (the habit of) remembering, and continually revolving in mind the articles of faith; as also that great profit is derived from all sacred images, not only because the people are thereby admonished of the benefits and gifts bestowed upon them by Christ, but also because the miracles which God has performed by means of the saints, and their salutary examples, are set before the eyes of the faithful; that so they may give God thanks for those things; may order their own lives and manners in imitation of the saints; and may be excited to adore and love God, and to cultivate piety. But if any one shall teach, or entertain sentiments, contrary to these decrees; let him be anathema.
The Church’s Code of Canon Law addresses the sale of relics point-blank: “It is absolutely forbidden to sell sacred relics.”