Gdansk is best visited between May and September. Summer (June–August) is warm and long, with temperatures around 20–24°C, and the waterfront terraces and amber markets along Mariacka Street are at their most lively. The Baltic coast does get rain even in summer, so a light jacket is always worth packing.
July and August bring the Dominican Fair (Jarmark Dominikanski), one of the oldest trade fairs in Europe, which has been running since 1260 and fills the old town with market stalls, food vendors, and street performers for three weeks. The Baltic Sail festival — an international gathering of tall ships — takes place in late July or early August and draws visiting crews and large crowds to the waterfront.
Spring (April–May) is quieter and pleasant, with the crowds not yet arrived. Autumn is similarly uncrowded and the light on the brick facades can be beautiful. Winter is cold and grey, but Gdansk does a decent Christmas market and the museums are good reasons to be indoors.