blue Monday

noun

a Monday regarded as a depressing workday in contrast to the pleasant relaxation of the weekend.

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for blue monday

Historical Examples of blue monday

Such reverses frequently occasion a "blue-Monday" in the middle of the week.
The Youthful Wanderer
George H. Heffner