By reanalysis of diurnus (“of the day”), which is from obsolete nominative diūs + -nus. Probably first spread to nocturnus (“of the night”) analogically, then to other terms.
-urnus (feminine -urna, neuter -urnum); first/second-declension suffix
First/second-declension adjective.
| singular | plural | ||||||
|---|---|---|---|---|---|---|---|
| masculine | feminine | neuter | masculine | feminine | neuter | ||
| nominative | -urnus | -urna | -urnum | -urnī | -urnae | -urna | |
| genitive | -urnī | -urnae | -urnī | -urnōrum | -urnārum | -urnōrum | |
| dative | -urnō | -urnae | -urnō | -urnīs | |||
| accusative | -urnum | -urnam | -urnum | -urnōs | -urnās | -urna | |
| ablative | -urnō | -urnā | -urnō | -urnīs | |||
| vocative | -urne | -urna | -urnum | -urnī | -urnae | -urna | |