In the evening of May 6, 1945, General Alfred Jodl arrived at the Western Allies' headquarters in Reims. His mission: surrender to the Allies, ending what became known as World War Two. He eventually signed the surrender treaty in the early morning hours of the following day.
But the Soviet leader Iosif Stalin could under no circumstances accept this. He insisted that the Germans officially surrender to the Soviets as well, which General Wilhelm Keitel did in Berlin on May 8 - May 9, Moscow time.
Part of the reason for this was pride; the USSR had borne the brunt of Allied casualties in fighting the Nazis. Stalin took credit for the Soviet war effort, and wanted the credit he considered his due. Another consideration, though, was more symbolic.
May 6 in the old Julian calendar still followed by the Orthodox faithful (whom the Communists had cruelly repressed) was the feast of St. George, the fabled dragon-slayer. In 1945, it was also the date of Easter. One can certainly understand why Stalin, as an atheist and Communist, would absolutely refuse to give that particular date any more meaning.
After the end of the war, Stalin had hundreds of thousands of liberated Soviet POWs either executed or sent to the gulag, so they would not be witnesses to his bumbling incompetence in 1941. He also turned on the victorious generals like Zhukov, jealous of their popularity. There was no room on the victor's pedestal for anyone but him. Especially not for God.
And so, May 9 became Victory Day in the Soviet Union, while the fact that the Germans surrendered on Easter became obscured by the fog of war.