tricennalia
The festival and religious rituals celebrating a Roman emperor's 30th year of rule.
historical
In 336, the city was ready. Constantine the Great could celebrate his Tricennalia, his thirty-year jubilee, in his new capital. One year later, he was baptised and died.
Where it comes from
From Latin trīcennālia, from trīcennium (“30-year period”) + -ālia (“-alia: forming the names of festivals”), from trīcennis (“30-year”) + -ium (“forming abstract nouns”), from trīciēs (“thirty each”) + annus (“year”) + -is (“forming compound adjectives”). Equivalent to tricennium + -alia.
Adapted from the Wiktionary entry tricennalia (CC BY-SA 4.0).