What declension does the word 'dies' belong to and how does it decline in both plural and singular states?

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.