The first verse from "Dies Irae" goes like
Dies irae, dies illa
I'm trying to understand what "illa" is referring to.
According to the declension table for pronouns, "illa" corresponds either to feminine nominative/ablative plural or neuter nominative/accusative plural.
This means "illa" definitely doesn't refer to "dies".
So I guess the only possibility is that it refers to "ira", but for some reason I feel something's off.