Green’s Dictionary of Slang

St Monday n.

a day off; also attrib.

1779		Nocturnal Revels I 64: As this was Saint Monday, there was a very respectable appearance [in court] of Barbers, Publicans, Butchers and Informers.
c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Saint Monday A Holiday most religiously observed by Journeymen Shoemakers, and other inferior Mechanicks: a Profanation of that day, by working, is Punishable by a fine, particularly among the Gentle Craft. An Irishman observed, that this saint’s anniversary happened every week.
1788		Grose Classical Dict. of the Vulgar Tongue (2nd edn) n.p.: [as 1786].
1801		Sporting Mag. July XVIII 170: Saint Monday [...] the real origin [of the term], every one knows, arises from the disposition of mechanics in general, always to follow up one holiday with another.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1841		Western Times 4 Sept. 3/4: It no doubt attracts a great many St. Monday folks into the town whose money is as good as any other body’s.
1856		Illus. Times (London) 19 July 14/2: St Monday, or the People’s Holiday.
1858		‘Shadow’ Midnight Scenes 37: ‘Saint’ Monday! Groups of idle workmen hand about tghe corners of the streets.
1863		Story of a Lancashire Thief 12: I never heard of him [...] getting bemused on Saint Monday.
1884		Daily News 22 July 5/3: It was evident that universal homage was being paid to Saint Monday. Working London proclaimed a general holiday [F&H].
1893		‘’Arriet on Labour’ in Punch 26 Aug. 88/1: Bit bosky, Sam, thick in the clear, as usual on Saint Monday.

In phrases

keep St Monday (v.)

to take a day off work.

1791		J. Lackington letter III Memoirs 18: While he was keeping Saint Monday, I was with boys of my own age, fighting, cudgel-playing, wrestling, etc.
1817		Chester Courant 16 Sept. 3/4: This [...] would only happen to those men who now invariably keep St Monday as a holiday [...] whether it was St Monady or St Saturday.
1828		North Amer. Rev. July 127: They work at it carelessly and negligently, just long enough to obtain a bare living, and then hurry [...] to the tavern to keep Saint Monday.
1839		Cottager’s Monthly Visitor Mar. 75: You and I used to keep saint Monday together, when we worked with Master Smart, and with the loss of three shillings for the day’s wages.
1843		Visitor 327/2: Instead of exercising such diligence and prudence, these men choose to keep ‘Saint Monday’ at the public-house.
1854		Parliamentary Papers 16 June 174: Nine-tenths of the people who go out on Monday are not the best regulated people; they are those who keep Saint Monday.
1862		Birmingham Dly Post 12 July 4/5: Our workmen do not lose time on Saturday, nor do they keep St Monday.
1870		Western Times (UK) 13 June 2/2: Those of the working class who ’Keep St Monday’ on principle, will only put in an appearance.
1871		Punch 8 July 3/2: And the British Working men, / As the lounging Upper Ten, / Shall as little be impelled to keep St Monday.
1878		F. Powell Bacchus Dethroned 50: Sunday incapacitates him for work at the commencement of the week, so that he is compelled to keep Saint Monday, and sometimes Saint Tuesday also.
1888		Woman’s World 154: She does not, as a rule, keep Saint Monday, or take part in strikes.
1891		Sheffield Dly Teleg. 14 July 7/5: The deplorable editor of one of the papers [...] had said they kept St Monday in Sheffield.
1902		Western Dly Press (Bristol) 13 June 3/8: Thirty years ago it was an institution of workmen to keep up ‘St Monday’.