Taken from: https://www.newadvent.org/cathen/14506a.htm
Tenebræ is the name given to the service of Matins and Lauds belonging to the last three days of Holy Week. This service, as the “Cæremoniale episcoporum” expressly directs, is to be anticipated and it should be sung shortly after Compline “about the twenty-first hour”, i.e. about three p.m. on the eve of the day to which it belongs. “On the three days before Easter“, says Benedict XIV (Institut., 24), “Lauds follow immediately on Matins, which in this occasion terminate with the close of day, in order to signify the setting of the Sun of Justice and the darkness of the Jewish people who knew not our Lord and condemned Him to the gibbet of the cross.” Originally Matins on these days, like Matins at all other seasons of the year, were sung shortly after midnight, and consequently if the lights were extinguished the darkness was complete. That this putting out of lights dates from the fifth century, so far at least as regards the night Office, is highly probable. Both in the first Ordo Romanus and in the Ordo of St. Amand published by Duchesne a great point is made of the gradual extinction of the lights during the Friday Matins; though it would seem that in this earliest period the Matins and Lauds of the Thursday were sung throughout with the church brightly illuminated (ecclesia omni lumine decoretur). On Friday the candles and lamps were gradually extinguished during the three Nocturns, while on Saturday the church was in darkness from beginning to end, save that a single candle was kept near the lectern to read by.
All this suggests, as Kutschker has remarked, that the Office of these three days was treated as a sort of funeral service, or dirge, commemorating the death of Jesus Christ. It is natural also that, since Christ by convention was regarded as having lain three days and three nights in the tomb, these obsequies should have come in the end to be celebrated on each of the three separate occasions with the same demonstrations of mourning. There can be no reasonable doubt that it was from the extinguishing of lights that the service came to be known as Tenebræ, though the name itself seems to have arisen somewhat later. The liturgist de Vert has suggested an utilitarian explanation of the putting out of the candles one by one, contending that the gradual approach of the dawn rendered the same number of lights unnecessary, and that the number was consequently diminished as the service drew to a close. This view seems sufficiently refuted by the fact that this method of gradual extinction is mentioned by the first Ordo Romanus on the Friday only. On the Saturday we are explicitly told that the lights were not lit. Moreover, as pointed out under HOLY WEEK, the tone of the whole Office, which seems hardly to have varied in any respect from that now heard in our churches, is most noticeably mournful–the lessons taken from the Lamentations of Jeremias, the omission of the Gloria Patri, of the Te Deum, and of blessings etc., all suggest a service cognate to the Vigiliæ Mortuorum, just as the brilliant illumination of the Easter eve spoke of triumph and of joy, so the darkness of the preceding night’s services seems to have been designedly chosen to mark the Church’s desolation. In any case it is to be noticed that the Office of these three days has been treated by liturgical reformers throughout the ages with scrupulous respect. The lessons from Jeremias in the first Nocturn, from the Commentaries of St. Augustine upon the Psalms in the second, and from the Epistles of St. Paul in the third remain now as when we first hear of them in the eighth century.
The Benedictine Order, who normally have their own arrangement of psalms and nocturns, differing from the Roman, on these three days conform to the ordinary Roman practice. Even the shifting of the hour from midnight to the previous afternoon, when no real darkness can be secured, seems to have been prompted by the desire to render these sublime Offices more accessible to clergy and laity. Already in the thirteenth century it seems probable that at Rome Tenebræ began at four or five o’clock on the Wednesday (see Ord. Rom., xiv, 82, and Ord. Rom., xv, 62). Despite the general uniformity of this service throughout the Western Church, there was also a certain diversity of usage in some details, more particularly, in the number of candles which stood in the Tenebræ hearse, and in some accretions which, especially in the Sarum Use, marked the termination of the service. With regard to the candles Durandus speaks of as many as seventy-two being used in some churches and as few as nine or seven in others. In England the Sarum Ordinal prescribed twenty-four, and this was the general number in this country, variously explained as symbolizing the twenty-four hours of the day, or the twelve Apostles with the twelve Prophets. A twenty-fifth candle was allowed to remain lighted and hidden, as is done at the present day, behind the altar, when all the others had been gradually extinguished. At present, the rubrics of the “Ceremoniale,” etc., prescribe the use of fifteen candles. The noise made at the end of Tenebræ undoubtedly had its origin in the signal given by the master of ceremonies for the return of the ministers to the sacristy. A number of the earlier Ceremoniales and Ordines are explicit on this point. But at a later date others lent their aid in making this knocking. For example Patricius Piccolomini says: “The prayer being ended the master of ceremonies begins to beat with his hand upon the altar step or upon some bench, and all to some extent make a noise and clatter.” This was afterwards symbolically interpreted to represent the convulsion of nature which followed the death of Jesus Christ.