Here are a few notes on the Latin for day—diēs.
1. Diēs is a fifth-declension noun.
2. Diēs is typically masculine (like most fifth declension nouns), but is occasionally feminine, especially in fixed phrases and general reference to time or dates.
- cōnstitūtā diē : on a fixed day
- longa diēs intervēnit : a long time had passed
3. Diēs is one of only two nouns in the fifth declension that is entirely declined. Rēs is the other such noun—all other fifth declension nouns are wanting in the plural (or at least the plural genitive, dative and ablative) in extant Latin literature.
The Essential AG: 96, 97, 98a