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.