semicentennial
noun
semi·cen·ten·ni·al ˌse-mē-sen-ˈte-nē-əl 
ˌse-ˌmī-,
-mi-
: a 50th anniversary or its celebration
semicentennial adjective
Example Sentences
Recent Examples on the Web An outdoor rock ’n’ roll show featuring local Wilmette bands took center stage Saturday night, all part of a celebration of the semicentennial of the Centennial Recreation Complex. 
Gina Grillo, Chicago Tribune, 15 Aug. 2022 This season, Wolf Trap in Vienna, Va., returns to the scene to celebrate the semicentennial of the Filene Center. Washington Post, 9 June 2021
Gina Grillo, Chicago Tribune, 15 Aug. 2022 This season, Wolf Trap in Vienna, Va., returns to the scene to celebrate the semicentennial of the Filene Center. Washington Post, 9 June 2021 Word History
First Known Use
1859, in the meaning defined above