Although the Church does not oblige us to believe in any particular prophecy or private revelation, it certainly is telling that many mystics and saints speak of the Three Days of Darkness, such as:
Blessed Anna-Maria Taigi, Ven. Elizabeth Canori-Mora, Saint Hildegard and many others.
Here are a few quotes from these mystics.
Blessed Anna Maria Taigi (1769-1837 A.D.):
"God will send two punishments: one will be in the form of wars, revolutions and other evils; it shall originate on earth. The other will be sent from Heaven. 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."
The mystic Marie-Julie Jahenny also mentions the 3 days of darkness. (January 4, 1884):
"The three days of darkness « will be on a THURSDAY, FRIDAY AND SATURDAY. Days of the Most Holy Sacrament, of the Cross and Our Lady. . . .» three days less one night."
"«The earth will be covered in darkness», says Our Lady on 20th of September 1882, AND HELL WILL BE LOOSED ON EARTH. Thunder and lightning will cause those who have no faith or trust in My Power, to die of fear."
"During these three days of terrifying darkness, no windows must be opened, because no one will be able to see the earth and the terrible color it will have in those days of punishment without dying at once... "
"The sky will be on fire, the earth will split... During these three days of darkness let the blessed candle be lighted everywhere, no other light will shine.... "
Sr. Rosa Colomba Asdenti of Taggia who lived in the 19th c. also prophesied a three days of darkness.
Palma Maria d' Oria (d.1863), as do other seers, tells of the activities of the demons:
"There shall be three days of darkness, during which the atmosphere will be infected by innumerable devils, who will cause the death of large multitudes of unbelievers and wicked men. Blessed candles alone shall be able to give light and preserve the faithful Catholics from this impending dreadful scourge.
Sr. Mary of Jesus Crucified of Pau, (d.1878):
"During a darkness lasting three days the people given to evil will perish so that only one-fourth of mankind will survive."