Some explanation on the significance of the ceremonies of Tenebrae held during the Sacred Triduum.
From the 1913 edition of The Catholic Encyclopedia, we offer some extracts about the significance of the Office of Tenebrae, a combination of Matins and Lauds, which is recited on the mornings of the Sacred Triduum (Holy Thursday, Good Friday and Holy Saturday).
...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 Tenebrae, though the name itself seems to have arisen somewhat later.
...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 Vigiliae Mortuorum [Vigil of the Dead], 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."