Quiz: Remember the definition of mal de mer?
Old English mondæg, monandæg "Monday," literally "day of the moon," from mona (genitive monan; see moon (n.)) + dæg (see day). Common Germanic (cf. Old Norse manandagr, Old Frisian monendei, Dutch maandag, German Montag) loan-translation of Late Latin Lunæ dies, source of the day name in Romance languages (cf. French lundi, Italian lunedi, Spanish lunes), itself a loan-translation of Greek selenes hemera. The name for this day in Slavic tongues generally means "day after Sunday."
Phrase Monday morning quarterback is attested from 1932, Monday being the first day back at work after the weekend, when school and college football games were played. Black Monday (mid-14c.) is the Monday after Easter day, though how it got its reputation for bad luck is a mystery. Saint Monday (1753) was "used with reference to the practice among workmen of being idle Monday, as a consequence of drunkenness on the Sunday" before [OED]. Clergymen, meanwhile, when indisposed complained of feeling Mondayish (1804) in reference to effects of Sunday's labors.