What is the origin of the idiom "Black Days" and when to use it?

The origin of the idiom "Black Days" can be traced back to significant historical events or tragedies that occurred on those particular dates. It is commonly associated with the stock market crash of 1929, which marked the beginning of the Great Depression. It can be used to commemorate historical events marked by significant losses or disasters, such as national tragedies, terrorist attacks, or natural disasters.