Since the end of the Second World War, the USSR, and its successor countries, especially Russia, Ukraine, and Belarus, have celebrated Victory Day on May 9.
Although the Germans surrendered to the Soviets on May 8 in Berlin, it was already early in the morning (12:43 am) in Moscow. And so, due to this time difference, those who celebrate the Soviet victory celebrate on May 9, not May 8.
At the same time the German surrender to the British and Americans occured towards the end of May 7, but hostilities were agreed to cease at 23:01 Central European Time on May 8. And so, the Americans and Europeans recognize May 8 as Victory Europe Day.
The Estonians' predicament is as interesting as always. You see, the Soviets set Estonian clocks to Moscow time when they reconquered this small land in 1944. That is, in Helsinki it was 11:43 pm when the Germans surrendered to the Soviets, but in Tallinn it was officially 12:43 am.
However, since most would agree that the sun reaches Tallinn and Helsinki after it reaches Moscow, you can surmise that the Estonians prefer to celebrate this event now at the proper time on May 8, rather than May 9. You could call it revisionism. Or you could just say that the timing of the commemoration is reality-based, as opposed to ideology-based. Either way, Estonia now celebrates the "end of the war" -- which dragged on into the 1950s in Estonia -- on May 8, not May 9.