Before answering the Question, here's what you need to know about this prophecy
Just as God punished the Egyptians with plagues including the “three days of darkness”, the theory states that God will chastise the world with darkness at the end of time. The earth will be enveloped by darkness lasting three days and three nights, the only light source that can be seen being blessed candles. Various Catholic visionaries agree that the faithful should stay within their homes during this period as most of the earth’s inhabitants shall die.
Many Catholic seers and mystics such as Saints Hildegard of Bingen, Gaspar del Bufalo, Padre Pio, Anna Maria Taigi, Elisabeth Canori Mora, Elena Aiello, Mariam Baouardi (Mary of Jesus Crucified), Rosa-Colomba Asdente, Palma d’Oria, Pere Lamy, Marie Martel, and Marie Julie Jahenny, have prophesied the Three Days of Darkness. However it is not endorsed by the Holy See.
Blessed Anna Maria Taigi (1769–1837) is the most known seer of the Three Days of Darkness and describes the event in this way:
There shall come over the whole earth an intense darkness lasting three days and three nights. Nothing can be seen, and the air will be laden with pestilence which will claim mainly, but not only, the enemies of religion. It will be impossible to use any man-made lighting during this darkness, except blessed candles. He, who out of curiosity, opens his window to look out, or leaves his home, will fall dead on the spot. During these three days, people should remain in their homes, pray the Rosary and beg God for mercy. All the enemies of the Church, whether known or unknown, will perish over the whole earth during that universal darkness, with the exception of a few whom God will soon convert. The air shall be infected by demons who will appear under all sorts of hideous forms.
Marie-Julie Jahenny (1850-1941), known as the “Breton Stigmatist”, expanded upon the story of the Three Days of Darkness, saying that it will occur on a Thursday, Friday and Saturday when all of Hell will be let loose to strike at those outside their homes and those without a lit blessed candle of 100% pure wax
Texts above from wikipedia
This is a private revelation of several Catholic saints and mystics. Whether this ‘darkness’ is to be taken literally or figuratively is a cause of many arguments. But we know that such threats of punishment from God is given for people to change from their sinful ways and repent. Therefore, such threat can in fact be averted if the appropriate reaction is met, so it’s debated whether God has in fact averted this punishment or whether it is still going to happen. The Church doesn’t have any official stand on this since there’s hardly a way to verify it’s authenticity (as coming from God). Therefore Catholics are free to speculate.