Not a saint at all, but ‘keeping Saint Monday’ was formerly used to describe the regular practice of staying off work on Mondays, particularly in the shoemaking trade. The custom was already well known in the 17th century, as evidenced by the line in the play: ‘They say Monday's Shooemaker's holliday, I'le fall to that trade’ (Dekker, If It Be Not Good, The Diuel Is In It (1612). This gives the lie to a legend, involving a Perth shoemaker and Oliver Cromwell, which seeks to explain its origin (Folk-Lore Record 1 (1878), 245–6). The custom fell into disuse following the gradual spread of more regulated working hours and the introduction of half-day working on Saturdays. There is a French phrase, reported from the 16th century, faire le lundi des savetiers, or ‘to keep the cobbler's Monday’.
N&Q 8s:1 (1892), 88, 212, 232–3, 252, 441, 523.