Ianuarius

• iānuārius, Jānuārius, jānuārius, Ian., Jan.

From Iānus (“Janus”) +‎ -ārius.

• (Classical Latin) IPA^(key): /i̯aː.nuˈaː.ri.us/, [i̯äːnuˈäːriʊs̠]
• (modern Italianate Ecclesiastical) IPA^(key): /ja.nuˈa.ri.us/, [jänuˈäːrius]

Iānuārius (feminine Iānuāria, neuter Iānuārium); first/second-declension adjective

1. of January

In Classical Latin, month names were regularly used as adjectives, generally modifying a case-form of mēnsis m sg (“month”) or of one of the nouns used in the Roman calendar to refer to specific days of the month from which other days were counted: Calendae f pl (“calends”), Nōnae f pl (“nones”), Īdūs f pl (“ides”). However, the masculine noun mēnsis could be omitted by ellipsis, so the masculine singular forms of month names eventually came to be used as proper nouns.^[1]

The accusative plural adjective forms Aprīlīs, Septembrīs, Octōbrīs, Novembrīs, Decembrīs^[2] are ambiguous in writing, being spelled identically to the genitive singular forms of the nouns; nevertheless, the use of ablative singular forms in -ī and comparison with the usage of other month names as adjectives supports the interpretation of -is as an accusative plural adjective ending in Classical Latin phrases such as "kalendas Septembris".^[3]

First/second-declension adjective.

Number	Singular				Plural
Case / Gender	Masculine	Feminine	Neuter		Masculine	Feminine	Neuter
Nominative	Iānuārius	Iānuāria	Iānuārium		Iānuāriī	Iānuāriae	Iānuāria
Genitive	Iānuāriī	Iānuāriae	Iānuāriī		Iānuāriōrum	Iānuāriārum	Iānuāriōrum
Dative	Iānuāriō		Iānuāriō		Iānuāriīs
Accusative	Iānuārium	Iānuāriam	Iānuārium		Iānuāriōs	Iānuāriās	Iānuāria
Ablative	Iānuāriō	Iānuāriā	Iānuāriō		Iānuāriīs
Vocative	Iānuārie	Iānuāria	Iānuārium		Iānuāriī	Iānuāriae	Iānuāria

Iānuārius m (genitive Iānuāriī or Iānuārī); second declension

1. January
2. a male given name

Second-declension noun.

Case	Singular	Plural
Nominative	Iānuārius	Iānuāriī
Genitive	Iānuāriī Iānuārī1	Iānuāriōrum
Dative	Iānuāriō	Iānuāriīs
Accusative	Iānuārium	Iānuāriōs
Ablative	Iānuāriō	Iānuāriīs
Vocative	Iānuārī	Iānuāriī

¹Found in older Latin (until the Augustan Age).

Borrowings

• Roman calendar on Wikipedia.Wikipedia

• “Iānuārius”, in Charlton T. Lewis (1891) An Elementary Latin Dictionary, New York: Harper & Brothers
• “Iānuārius” on page 817/1 of the Oxford Latin Dictionary (1st ed., 1968–82)