vicennalia
The festival and religious rituals celebrating a Roman emperor's 20th year of rule.
historical
Apart from the Vicennalia, 136 was an eventful year for the imperial family: the serious illness of Hadrian in the spring, the adoption of Aelius in the summer and the death and consecration of Sabina at the end of the year.
Where it comes from
From Latin vīcennālia, from vīcennium (“20-year period”) + -ālia (“-alia: forming the names of festivals”), from vicennis (“20-year”) + -ium (“forming abstract nouns”), from vīciēs (“twenty each”) + annus (“year”) + -is (“forming compound adjectives”). Equivalent to vicennium + -alia.
Adapted from the Wiktionary entry vicennalia (CC BY-SA 4.0).