Definify.com
Webster 1913 Edition
Black Monday
Black′ Monˊday
.1.
Easter Monday, so called from the severity of that day in 1360, which was so unusual that many of Edward III.’s soldiers, then before Paris, died from the cold.
Stow.
Then it was not for nothing that my nose fell a bleeding on
Black Monday
last. Shakespeare
2.
The first Monday after the holidays; – so called by English schoolboys.
Halliwell.