At the time of writing, we in Western Europe and also the USA are still on May the 8th, the day we celebrate Victory in Europe Day, when it was announced that the Germans had surrendered to the Allies and thus the war in Europe was over. 1 day before, the Germans had signed their Instrument of Surrender, the legal document in Reims where it was stated that fighting was to stop at 23:01 CET of the 8th. This is the main reason why the 8-May is considered the day for people in CET (Central European Timezone) or earlier (further West), because the war stopped officially on that day.
However it was already the 9-May in the Soviet Union when this occurred, and adding to this, the Soviet High Command didn't fully agree with the signing in Reims as they claimed it had to happen at the seat of Germany's government, Berlin. So another gathering occurred on the night of the 8th and a new version of the Instrument of Surrender was signed, the last signature being put to paper on the 9-May, 00:16 local time. These are the main reasons that Russia is celebrating the Victory Day of the Great Patriotic War (as they call WWII) tomorrow, Monday the 9-May, and not today with the rest of the Allies (USA, UK and France, the other countries that alongside USSR controlled a section of Germany after this).