AHA!!! Paden's Old Occitan (1998), Chapter 22 "Nouns and Adjectives (Section 1)" explains in a parenthetical comment about the declension of the Old Occitan (OOc) "jorns" ("day") and how it compares in declension to its Classical Latin (CL) "diurnus":
Note that in Classical Latin, DĬŬRNUS was an adjective meaning 'daily,' not a noun; in VL [Vulgar Latin] the adjective replaced CL DĪĒS, of the unusual fifth declension.
In other words, "diurnus" was appropriated and nounified as a second declension noun by medieval Latinophones because, if I understand Paden right, they were such slackers that they wanted out of dealing with fifth declension nouns. Which I am totally sympathetic with. By the time you make it through learning the third declension, you're like, "Screw this. There's a first or second declension synonym somewhere."
So presumably words that descend from "diurnus"-as-a-noun (such as OOc "jorns", It "giorno", Fr "jour", Eng "journey" and "journal") are later forks off the Latin tree than words that descend from "dies".
ETA: At least according to modern Occitan, the days of the week are also descended from dies, not diurnus: diluns, dimars, dimècres, dijóus, divendres, dissabte, & dimenge.
And according to my source for that, the answer to the obvious question is: lunae dies, martis dies, mercurii dies, iovis dies, veneris dies, sabbatum, et dominicus.
(no subject)
So presumably words that descend from "diurnus"-as-a-noun (such as OOc "jorns", It "giorno", Fr "jour", Eng "journey" and "journal") are later forks off the Latin tree than words that descend from "dies".
ETA: At least according to modern Occitan, the days of the week are also descended from dies, not diurnus: diluns, dimars, dimècres, dijóus, divendres, dissabte, & dimenge.
And according to my source for that, the answer to the obvious question is: lunae dies, martis dies, mercurii dies, iovis dies, veneris dies, sabbatum, et dominicus.