The word dies is a fifth declension noun and as such is regarded to be feminine.
In the order of nominative, accusative, genitive, dative then abalative cases in the singular form, dies declines as follows: dies, diem, diei, diei, die.
In the same order as above but for plural it declines as follows: dies, dies, dierum, diebus, diebus.
In the vocative case dies presents in both singular and plural form as dies.