Definition of black monday:
(n) :
(colloquial) The first Monday back at school after the holidays.
(n) :
Any of certain Mondays when undesirable or turbulent events have occurred.
(n) :
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.