Like Jeff said, the most natural way to ask this in American English is to say "When did Germany declare war on Russia?", but you could also ask "On what date did Germany declare war on Russia?" If you ask "On what date did Germany declare war on Russia?" , you are asking for the exact date. So the listener would have to respond with "August 1st, 1914". If you ask "When did Germany declare war on Russia?" , the listener should also respond with "August 1st, 1914" but they may be lazy and just respond with "in 1914" instead. Hehehehe

How about "When did Germany declare war on Russia? " or "On which day did Germany declare war on Russia? " or "What is the exact date of Germany declaring war on Russia?"