quindecennalia

plural quindecennalias

1. historicalThe festival and religious rituals sometimes observed celebrating a Roman emperor's 15th year of rule.

   On 1 January 112... the year opened with a series of celebrations to mark the quindecennalia of the emperor's ascension.

   1997, Julian Bennett, Trajan Optimus Princeps: A Life and Times, page 186

   2 more examples

   There is evidence, that the quindecennalia, of fifteenth anniversary of Trajan's proconsular authority, and of his adoption to the empire, was celebrated in 111, or the year after Pliny's arrival in Bithynia.

   1837, Edward Burton, Lectures upon the Ecclesiastical History of the First Three Centuries..., Vol. II, "Lecture XIV", p. 40

   There is good literary evidence for Honorius' tricennalia in 422, which is at the end of perfect quinquennial sequence from 393, as well as consular vot xxx mvlt xxx solidi, though there do not appear to be any vota coins marking his quindecennalia or quinta quinquennalia.

   1988, Richard W. Burgess, “Quinquennial Vota and the Imperial Consulship in the Fourth and Fifth Centuries, 337-511”, in Numismatic Chronicle, number 148, page 85