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.^[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]