World War 2 (or Great Patriotic War) end was celebrated on May 9th in USSR (and in some other countries of the socialist block, e.g. Poland), and is celebrated on that date in all post-soviet countries.
I've heard two explanations for this:
It was after midnight Moscow-time when the surrender was signed on May 8th in Berlin, so it is technically ended on May 9th for Soviets.
Stalin wanted to separate Great Patriotic War from World War 2 as much as possible, and wanted their own special celebration, separate from the rest of the world.
Which of these explanations is more likely to be correct? Is there any other explanation?
I've read that the Russian diplomat present at Riems (when the West signed the treaty) was not authorized by Stalin to sign it, so on May 8th Zhukov and Kietl signed the treaty. As you said, when that treaty was signed it was already May 9th in the Ussr.