In American literature of the 1830s, Blue Monday referred to the hungover state of the labor workforce after a weekend spent drinking, and the association of the color
blue
Blue is one of the three primary colours in the RYB colour model (traditional colour theory), as well as in the RGB (additive) colour model. It lies between violet and cyan on the spectrum of visible light. The eye perceives blue when ...
with a
depressed state of mind.
In the 1860s, the term began to be applied to a weekly home "wash day." White clothing was sometimes rinsed with
bluing, however the relevance of this fact to the nickname for Mondays is contested, and the name may simply carry over from the earlier usage.
[What is bluing?, http://www.victorianpassage.com/2009/11/what_is_bluing.php]
References
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English language
English_words_and_phrases
Monday