Today is known as Black Friday in the US, a very busy shopping day traditionally marking the start of the Christmas shopping season. I heard someone on the radio this morning saying that Black Friday is so called because this is the day that shops' bank accounts go from being in the red to being in the black. So, it is a positive epithet.
This explanation is also given in the OED, but the Dictionary also tells us that it was Philadelphia police officers who first referred to the day as Black Friday. They did so because of the huge numbers of shoppers and huge amount of traffic they were faced with on this day, giving them lots of headaches (they referred to Black Saturday, too). The OED's first citation for this sense is from a 1961 journal.
Black Friday has referred to other days and events in the past. The earliest usage of the term was in the early 17th century and was school slang for an exam day. Friday 6th December 1745 was referred to as Black Friday, too. This was the date on which the landing of the Young Pretender was announced in London (the Young Pretender is also known as the romantic figure of Bonnie Prince Charlie, a grandson of the Stuart king James II, who pretended to the throne of Great Britain and Ireland as, indeed, his father, the Old Pretender, had done before him).
On Friday 11 May, 1866, there was commercial panic in the City of London after the failure of the London bank Overend, Gurney & Co (very coincidentally, my last post before this one was on the Gurney bank - spooky! I only heard the name for the first time yesterday and have now come across it again barely 12 hours later). This date, too, has been described in the press up to the present day as Black Friday. The USA had their own Black Friday a few years later -- on Friday 24 September 1869; panic on Wall Street was precipitated by flooding the market with government gold.
Belated Thanksgiving and holiday greetings to readers in the US.