Kalendae and Nonae, pl. tantum

Quare sunt Kalendae et Nonae plurale tantum?

The easy answer is “because they are”, same with idus, iduum. I don’t know the actual etymological reason, but my best guess would be that the words refer to all of the events of the days rather than the days themselves. The more perplexing question to me is why they are feminine - there must be some noun that nonae and probably even kalendae once agreed with, and I can’t imagine it being res

But are they adjectives?

Horae?

I would think that kalendae is a gerundive, and nonae is an adjective, but idus (being fourth declesion) goes against the grain.

< divido