The term "Blue Monday" was long a
nickname for
Mondays aboard
English ships. Centuries ago,
sailors found
guilty of misdeeds were
flogged on this
dreaded day, often "until
black and blue." The
Royal Navy did not formally
abolish this practice until
1948.
While mostly a
feared day, it often was also a day of
mischief and
disobedience on these same ships, as sailors who were
convicted of minor
infractions on a Monday would often see their
punishments reduced or even eliminated by the time the next "
Blue Monday" would roll around. Still, these floggings were often enough to keep
order on the
British vessels for the most part, especially on long voyages where
infection and
death were
common consequences of even the most light of floggings.
Source: Frank Bowen's Sea Slang: A Dictionary of the Old-Timers' Expressions, 1929.