disaster: a sudden, crushing catastrophe that overtakes a person or a nation, often the appointed day of their downfall
This word names ruin that comes down hard and fast. It is not everyday bad luck or ordinary trouble; it is the catastrophe that overturns a life or a nation, the kind of blow that rises up suddenly and leaves the victim undone. Because it arrives out of nowhere and levels everything, English Bibles reach for strong words to catch it: calamity, disaster, destruction, distress, and ruin.
Much of the time the word carries a note of doom that is coming or already reserved. A common pattern is the phrase "the day of their calamity," a fixed day of reckoning that hangs over the wicked or over a doomed nation and finally lands. In the wisdom books it is the downfall waiting for those who do evil, ready at their side. In the prophets it is the catastrophe God brings on Moab, on Edom, on Egypt when their time is up.
The same word also names the dark day a righteous sufferer can fall into. When David says his enemies confronted him "in the day of my calamity," he means the day disaster closed in on him. Whether the sufferer deserves it or not, the sense holds steady: a sudden, overwhelming catastrophe.