Template:U:la:month

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.

The accusative plural adjective forms Aprīlīs, Septembrīs, Octōbrīs, Novembrīs, Decembrīs 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".