dies

Dies is the model noun of the fifth declension: dies, diei, diei, diem, die, with the plural dies, dierum, diebus. It is normally masculine, but it turns feminine when it means an appointed day or a stretch of time, which is why dies constituta above carries a feminine ending. The bare ablative die is the standard way to say when something happened, as in tertio die, 'on the third day'. English diary, dial and diurnal all descend from it.