SARAJEVO, Bosnia-Herzegovina (AP) — Opposition leaders in the predominantly Serb region of Bosnia are vowing to hold a referendum on re-establishing an official holiday after an unfavorable ruling from the country's constitutional court.

The Sept. 25 referendum will ask residents of Bosnia's Republika Srpska whether Jan. 9 should remain the region's annual day.

Bosnia's constitutional court ruled earlier this year that the date is disrespectful to Bosniaks and Croats in the region since it falls on the same day as a Serb Orthodox religious holiday.

Opposition leader Mladen Ivanic said Friday the date is a matter of Serb "national pride." It marks the wartime establishment during the 1990s of a Serb-only region within the once multi-ethnic Bosnia.

International officials overseeing the peace agreement in Bosnia are opposing the referendum. Russia, however, is supporting it.