Summer (June to August) is the best time, with the Curonian Spit beaches at their most accessible and the old town at its liveliest. July temperatures average around 20°C; the weather is pleasant though never hot. The Sea Festival in late July is the largest in Lithuania and worth timing a trip around.
Spring and early autumn are good for quieter visits. Winter is cold (regularly below freezing) and the Curonian Spit is largely closed, but Klaipeda itself remains functional and the Christmas market adds some cheer in December.