Kronstadt is worth visiting year-round, but summer — June through August — is the best time. The boat trips to Fort Alexander operate only in summer (typically May through September), and the island has a warmth and colour in good weather that makes the walk from the cathedral to the harbour particularly pleasant. The White Nights period (late June/early July) gives long evenings to explore.
Winter is cold and windy — Kotlin Island is exposed in the Gulf of Finland and can feel considerably more Arctic than St Petersburg itself. The cathedral and museums are open year-round. Spring and autumn are pleasant but often grey. Kronstadt is not a major tourist destination, so there is never a problem with crowds.