semimillennial

1. not comparableOf or related to a 500th anniversary.

   On the occasion of the semimillennial anniversary of Uppsala University, Twelfth Symphony by the romantic Swedish composer Allan PETTERSSON, entitled De Döda på Torget (The Dead in the Marketplace) for chorus and orchestra […] is performed for the first time by the Stockholm Philharmonic.

   2001, Laura Kuhn, Music since 1900, 6th edition, New York, N.Y.: Schirmer Reference, →ISBN, page 689

   Synonyms: quincentennial

plural semimillennials

1. A 500th anniversary.

   At a casual glance [Karl] Jaspers’ words seem to deprecate research, strange doctrine to preach at the semimillennial of an institution of learning as distinguished as the University of Basel.

   1969/1970 Winter, Gerald W. Johnson, “Program for the Mini-Intellectual”, in Hiram Haydn, editor, The American Scholar, volume 39, number 1, Richmond, Va.: Phi Beta Kappa, page 50

   Synonyms: quincentennial