Green’s Dictionary of Slang

1. a Monday taken off work and dedicated to self-indulgence [blue adj.² (1)].

1862		Yankee Volunteer’s Songster 51: Uncle Sam, the son of Johnny Bull, the boss, / On Blue-Monday took a spree, sirs [DA].
1885		Harper’s Mag. 873/1: The workman getting sober after his usual blue Monday [F&H].
1885		Dly Teleg. (Monroe, LA) 30 Dec. 4/2: Blue Monday. It used to be a custom in many countries for the journeyman and laborer to consider every Monday a day set apart for idleness.
1896		Eagle (Silver City, NM) 22 jan. 7/1: He overlooked the fact that the meeting of tghe bureau was called on the 13th of the month and on blue Monday at that.
1915		Day Book (Chicago) 25 Sept. 24/2: No wonder the saloon and the church go hand in hand. No wonder we have blue Monday and everybody drunk on Monday after listening to sermons on hell [...] on Sunday.

2. a start-of-the-week feeling of depression that follows a weekend of pleasurable excess [blue adj.¹ (1)].

1904		Camden Chron. (TN) 4 Mar. 6/4: More women commit suicide on Monday than on any other day [...] ‘Blue Monday’, as it has long been called, is one of the most trying days [...] because it is a ‘wash day’.
1919		Eve. Public Ledger (Philadelphia, PA) 16 June 16/5: Oh, dear! Another monday — Blue Monday!
c.1950		Eddie Mack ‘Seven Day Blues’ 🎵 Why do they call it blue Monday when every day is just as blue?
1965		J. Thompson Texas by the Tail (1994) 125: Blue Monday – a hard morning after a hard weekend. It figured.