Black Monday

English

Any of certain Mondays when undesirable or turbulent events have occurred.
March 9, 2020, in the midst of the 2020 stock market crash, which resulted from market instability due to the COVID-19 pandemic. It was followed three days later by a similar event called Black Thursday.
(colloquial) The first Monday back at school after the holidays.
- 1749, Henry Fielding, The History of Tom Jones, a Foundling, volume (please specify |volume=I to VI), London: A[ndrew] Millar, […], →OCLC:
  [H]olidays were my most unpleasant time; for my mother, who never loved me, now apprehending that I had the greater share of my father’s affection, and finding, or at least thinking, that I was more taken notice of by some gentlemen of learning, and particularly by the parson of the parish, than my brother, she now hated my sight, and made home so disagreeable to me, that what is called by school-boys Black Monday, was to me the whitest in the whole year.
- 1857, Thomas Hughes, Tom Brown's School Days:
  Now, in my time, when we got home by the old coach, which put us down at the cross-roads with our boxes, the first day of the holidays, and had been driven off by the family coachman, singing "Dulce Domum" at the top of our voices, there we were, fixtures, till black Monday came round.

(first Monday back at school): 1873, John Camden Hotten, The Slang Dictionary