Plymouth's climate is mild and maritime — wetter than inland Devon but rarely cold enough for frost. Summer (June to August) brings the most visitors and the warmest weather, but the city works well year-round. The Barbican is lively in summer evenings; winters are quiet but the naval history and museums give you plenty to do indoors.
The British Firework Championships take place in Plymouth Sound in August, with competing teams launching displays from barges anchored in the water — the Hoe is the best viewpoint and draws enormous crowds. Plymouth Seafood Festival runs in September and showcases the local fishing industry.
Spring and early autumn are arguably the best times: the summer crowds are absent, the light is good, the sea is calm enough for boat trips, and accommodation prices are lower. Avoid the week of the school summer holidays (late July/early August) if you want the Barbican to yourself.