Two historically important events on 30th April make it significant.
One is the down fall of the Fascist Hitler. He started the II World War in 1942 invading and conquering other Europen Countries. Later he attacked the Soviet Union. But the Soviet Red Army after much casualities turned from defence to offence and kicked back the Nazi forces back to Germany. The Red Flag was hoisted in Berlin on 2nd May 1945 heralding the end of war for Germany. When Hitler understood he has lost he committeed suicide on 30th April 1945 itself. The tragic end of the Fascist who threatened the world.
The second is the liberation of Vietnam from the clutches of the US imperialist vultures. After sustained and determined guerilla warfare by the Vietnamese, the highly sophisticated and brutal army of the US was utterly defeated and Vietnam liberated on 30th April 1975, 30 years after fall of Hitler.
Both incidents are liberation for the people. Both had a significant role in history.