Russia's climate varies enormously across its vast territory, but most travelers will be visiting Moscow, St. Petersburg, and the Golden Ring, where the seasons are distinct and dramatic.
Summer (June-August) is the most popular time to visit. Days are long -- St. Petersburg's famous White Nights in late June bring nearly 24 hours of daylight. Temperatures in Moscow and St. Petersburg range from 20-30C. This is the best time for the Trans-Siberian Railway and Lake Baikal, though mosquitoes in Siberia can be fierce in July.
Autumn (September-October) brings golden colors and fewer crowds. September is particularly pleasant in Moscow and St. Petersburg, with mild temperatures and clear skies. By late October, temperatures drop sharply.
Winter (November-March) is cold -- seriously cold. Moscow averages -10C in January, and Siberian cities can drop below -40C. But winter has its own magic: snow-covered onion domes, frozen rivers, the Northern Lights in Murmansk, and far fewer tourists. Dress in layers and be prepared for the cold. Lake Baikal freezes solid and can be crossed on foot or by car.
Spring (April-May) is muddy and unpredictable. The snow melts, roads flood, and everything looks grey. Late May improves, with warmer days and the start of the White Nights approaching. It is not the best time to visit unless you are heading to southern Russia.
The best months for most travelers are June through September. If you want the White Nights experience in St. Petersburg, aim for mid-June to early July.