vicennial

From vicennium +‎ -al or directly from Latin vīcenniālis, from vīcennium (“20-year span”) + -ālis (“-al”, adjective-forming suffix), from vīcennis (“20-year”) + -ium (“-ium”, suffix forming abstract nouns), from vīciēs (“20 times”) + annus (“year”) + -is (suffix forming compound adjectives). Equivalent to vice- (“twenty”) + -ennial.

vicennial (not comparable)

1. Occurring once every twenty years.
2. Lasting for twenty years.

• vigintennial

• annual (yearly), biennial (2), triennial (3), quadrennial (4), quinquennial (5), sexennial (6), septennial (7), octennial (8), novennial (9), decennial (10), centennial (100), bicentennial (200), quincentennial (500), millennial (1000), decamillennial (10,000), perennial (forever)

vicennial (plural vicennials)

1. A twenty-year period
   • 2022, Ira M. Sheskin, et al, “United States Jewish Population 2020”, in American Jewish Year Book 2020, page 176:

     The maps are presented at 20-year intervals (vicennials) because of the nature of the data, particularly for years prior to 1960 when estimation procedures were not nearly as developed as today.

2. A twentieth anniversary, particularly
   • 1909 June, John H. Vincent & al., "'89's Vicennial", The Chautauquan, Vol. 55, No. 1, p. 130:

     Among the well established anniversaries celebrated by Chautauqua classes from year to year are the vicennials. A strong sense of comradeship is developed among members who come to Chautauqua twenty years after graduation...

   1. (historical) Synonym of vicennalia, the festival and religious ritual held in the 20th year of a Roman emperor's reign.
      • 1776, Charles le Beau, The History of Constantine the Great, page 267:

        The feast of Constantine's vicennials fell on the 25th of July this year: it was the commencement of the twentieth year of his reign. It is supposed, that to avoid interrupting affairs or greater moment, this ceremony was deferred till the end of the council, which broke up the 25th of August.

      • 2022, Martin Hallmannsecker, Roman Ionia..., p. 110:

        Roman coins mentioning the vota publica for Macrinus are RIC IV.1 5–13; on vota publica for the decennials and vicennials of emperors in the 2nd and 3rd c. ad, see Chastagnol 1984.

In reference to the Roman festival, sometimes used in the plural to match Latin vicennalia and provide for the variety of events.

• (Roman festival): decennial, tricennial

• “vicennial, adj.”, in OED Online ⁠, Oxford: Oxford University Press, launched 2000.