diei

Diei is the genitive singular form of the Latin noun dies, meaning "day." Dies is a fifth-declension noun that can denote a calendar day or, in plural, days in general. The standard paradigm is: nominative dies, genitive diei, dative diei, accusative diem, ablative die; plural nominative dies, genitive dierum, dative diebus, accusative dies, ablative diebus. Thus diei commonly appears where a particle means "of the day" or "to/for the day," depending on case.

In usage, diei is most often encountered in phrases that express temporal relationships. For example, hora diei means "the hour of the day" (the day’s hour), and eius diei can mean "of that day." The genitive diei can also appear in various compound expressions and in legal or formal Latin to indicate temporal reference. The dative singular diei is also a possible morphological form, though it is less frequently singled out in modern references beyond noting its existence in the declension.

Diei should not be confused with other uses of the English word "die" or with the plural form dies in contexts where the nominative plural is intended. In Latin inscriptions and classical prose, dies and its forms—including diei—appear across religious, legal, and historical texts, often in tight, formulaic phrasing to denote dates, periods, or temporal relations.

See also: Latin declensions, dies, Germanic and Romance date terminology, Latin phrases indicating time.