The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
Gender in the Fifth Declension.
97. All nouns of the Fifth Declension are Feminine,
except diés (usually M.),
day, and merídiés
a. Diés is sometimes feminine in the
singular, especially in phrases indicating a fixed time, and regularly
feminine when used of time in general as, cónstitútá dié,
on a set day; longa
diés, a long time.