Russia, as you know, is a very cold country with a continental climate, typically characterized by long winter and short summer. 
During summer it often gets pretty hot. In Moscow, for example, it is not surprising to have summer temperature above 30C (86F). 
Russia is a largest country in the world, with eleven time zones, and it is almost twice as big as USA . 
As you understand climate can be different in different areas of the country. 
It is softer near the Black Sea, this area has mild winters but it has a lot of rain all the year round. 
In the central areas of Russian, the difference of temperature between winter and Summer is huge, 30+ C in summer, and -30 C in the Winter. 
So what is the best time to visit Russia ? 
Your best choice would be to plan your trip in early Fall. 
September is a beautiful month in almost all Russian territory. The temperature is not that hot and not yet cold. 
October is a rain season and from November to April Russian weather is very cold. 
But if you like snow and not afraid of cold temperature, then you will love Russian winter! 
Nature looks as beautiful as in a fairy tale. 
Early spring bring a lot of mud and rain, but later spring and early summer are very nice months to visit Russia.