semi·quin·cen·ten·ni·al
ˌse-mē-ˌkwin-sen-ˈte-nē-əl
ˌse-ˌmī-,
-mi-
plural semiquincentennials
: a 250th anniversary or its celebration
July 4, 2026 marks the nation's semiquincentennial—its 250th birthday since the signing of the Declaration of Independence.—
Kathryn Palmer
Hanover, like some of the dozen or so New Hampshire towns celebrating their semiquincentennial [in 2011], started planning for the celebration back in early 2010.—
Dan Tuohy
: relating to or associated with a 250th anniversary or its celebration
semiquincentennial events/festivities
As the United States approaches its semiquincentennial celebration, Utah drivers will be able to purchase, for a limited time, a license plate with a fresh new design.—
Hayden Madson
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Merriam-Webster unabridged



