Operation Flash was launched at dawn of May 1, 1995 and was completed the next day. During the 31 hours, Croatian forces managed to retake 800 square kilometres of land from rebel Serb forces and regain control of the Zagreb-Lipovac highway.
About 16,000 Croatian soldiers and police officers were involved in the lightning offensive, 42 of whom were killed and 162 wounded.
The operation enabled the start of reconstruction of the Western Slavonia region and return of people displaced by war.
In retaliation for the defeat, Serb insurgents fired missiles with cluster munitions on Zagreb on May 2 and 3, 1995, killing six people and wounding 175, of whom 39 seriously. The Hague-based International Criminal Tribunal for the former Yugoslavia (ICTY) later indicted Serb rebel leader Milan Martic for ordering the attacks.
Central commemorative ceremonies with top state officials will be held in the town of Okucani on Saturday morning.