St. Mariam Baouardy, who was canonized yesterday is reputed to have joined those saints and mystics who have prophesied the three days darkness. I've never been able to find her prediction in the literature I have on the saint. I've only seen the quote on sites which promote prophecies on the warning and chastisement.
"All states will be shaken by war and civil conflict. During a darkness lasting three days the people given to evil ways will perish so that only one-fourth of mankind will survive." - St. Mary of Jesus Crucified.