In an answer to this question on Ζεύς Draconis mentions (quoting his own post on another site):
The accusative form of the root, *dyēm (cf AGrk Zēn), also survived in Latin, in the form *diēm > diem. Uniquely, the dy here didn't become *dj > j; some scholars suggest that it evolved in a different Italic language/dialect, which didn't have the palatalization rules, then got borrowed back into Latin. When it did come back to Latin, though, it was with a new meaning of "day" (instead of "sky/sky god"), and it was extrapolated into a whole paradigm on the stem di-, and forced into the fifth declension where it didn't really fit (di-ēs, di-ēī, etc).
The very last mention caught my attention: dies was forced into the fifth declension where it didn't really fit. What does this mean exactly?
What was the original declension of dies in Latin like? How does it compare to third and fifth declensions at the time?
What kinds of words were in the original fifth declension? How does their declension differ from that expected for dies? The whole declension cannot be modeled after dies if it is an outlier of some kind.
Do we know why dies ended up in the fifth declension? Do we know of a mechanism that would do so or are there forms that scream fifth declension?
This all is of course under the assumption that the statement by Draconis is reasonably accurate. If it is not, then arguing to the contrary would make a fine answer here.