Victory Day or 9 May marks the capitulation of Nazi Germany to the Soviet Union in the Second World War (also known as the Great Patriotic War in the Soviet Union and all post-Soviet states). It was first inaugurated in the fifteen republics of the Soviet Union, following the signing of the surrender document late in the evening on 8 May 1945 (after midnight, thus on 9 May, by Moscow Time). It happened after the original capitulation that Germany earlier agreed to the joint Allied forces of the Western Front. The Soviet government announced the victory early on 9 May after the signing ceremony in Berlin. Though the official inauguration happened in 1945 (which means it has been celebrated since 1946), the holiday became a non-labour day only in 1965 and only in some of the countries.

http://en.wikipedia.org/wiki/Victory_Day_%289_May%29