A dark day is not a metaphor. It is a rare natural phenomenon in which the light of the Sun fails for some reason to reach the Earth, and what should be daylight becomes darkness. Day darkness can vary in intensity from gloom to total blackness.
Dark days are among the earliest recorded natural events. The darkness of Exodus, which lasted three days, is an example. Another is the "cloud" which caused the Sun to disappear while the King of the Persians was besieging Larissa in c. 556 BC. After the death of Julius Caesar in 44 BC, the Sun was dimmed for a whole year, and the air was cold and misty. The darkness at the Crucifixion, which lasted only an afternoon, remains a mystery.
Dark days have a number of causes, some prosaic, others mysterious.
Obviously clouds can hide the Sun, and the thicker the cloud, the darker the gloom. However, darkness in daylight is so unsettling that, even when the cause is evidently a cloud, it can still cause alarm. On April 12, 1910, a darkness settled for two hours on Chicago, causing widespread terror among people who thought it was something to do with Halley`s Comet. The Weather Department said it was a combination of rain, wind and smoke. On April 30, 1971, an ominous overcast at Jacksonville, Florida, blotted out almost all daylight for half an hour. Streetlights came on and birds went to roost. There was no thunder and no rain, but there was a line of thunderstorms just north of the city.
A common winter feature of London and other large cities in the days before smokeless zones was what was called, euphemistically, "high fog". During an anticyclone, an inversion formed over the city, trapping fog and smoke beneath it. The Sun turned yellow, then red, and sometimes disappeared. On October 24, 1933, there was "midnight at mid-day" in London, when a "high fog" caused total darkness. An Imperial Airways pilot said that "the pall over London at mid-day looked like a huge black mushroom completely shrouding the city".
Smoke from burning forests and other large fires can cause darkness hundreds of miles away. This is the type of dark day that often causes terror, as it can happen in fair weather, with no obvious cause. The most famous example is the New England Black Friday of May 19, 1780. Candles were lighted at noon in Providence; in N.E. Massachusetts print could not be read outdoors for several hours; in Worcester, "a sickly melancholy gloom overcast the face of Nature". Soot-coloured rain fell, and this was enough to reveal the cause of the darkness to most people. A Massachusetts farmer disagreed, saying that to attribute the darkness to the "smoke of burnt leaves" was absurd; it was time to repent, as "the day of the Lord draws nigh".
Sometimes desert dust raised by a storm is lifted into the upper air, and, like smoke, carried for hundreds of miles. An event of this sort caused terror in Baghdad on May 20, 1857. A dust storm had been blowing all day, when at about 5 pm, a darkness set in that was "deeper than the darkest night". Panic gripped the city. "People looked for each other, to see the end of the world together." After about five minutes, "the black darkness was succeeded by a red, lurid gloom" and "a dense volume of red sand fell".
The most well-known example of this type of darkness is that described by Pliny the Younger, who was trapped near the erupting Vesuvius on August 24, AD 79. He said that at one point the darkness was "not the dark of a moonless or cloudy night, but as if the lamp had been put out in a closed room." People trying to escape the volcano shouted and screamed, in an effort to recognise each other by their voices.
A great volcanic eruption can send dust into the stratosphere and around the world, causing solar obscurations over a vast area. In AD 536, in Italy, "for the entire year the Sun sent forth his rays without his usual brilliance, like the Moon". In Mesopotamia, the Sun was darkened for 18 months. "Each day it shone for about four hours, and still this light was only a feeble shadow…the fruits did not ripen and the wine tasted like sour grapes". This may have been the result of a huge eruption from the New Britain volcano Rabaul. (There is a possibility that it was the result of an asteroid strike in South America, whose craters have recently been discovered).
All these dark days have been the result of obscurations within the Earth's atmosphere. There are some dark days, however, that may be caused by agents beyond the Earth.