The 3 Days of Darkness stems from private revelation of many saints and mystics. According to this prophecy, the world will experience three days of complete darkness, during which time demons will roam the earth and the faithful will be called upon to endure great suffering and trials. I believe this will occur at the near end of tribulation. https://afterthewarning.com/the-illumination/after-the-warning/the-three-days-of-darkness/

If you are digging, you may also want to look into the Illumination of Conscience. https://www.michaeljournal.org/articles/roman-catholic-church/item/the-warning-or-the-illumination-of-conscience