Sopot is emphatically a summer resort. The season runs from late June to mid-September, when the beach fills, the terraces open, the pier is busy, and the Forest Opera stages its concerts. This is when Sopot is at its most vibrant — and also its most crowded and expensive. Accommodation is booked weeks in advance during July and August.
Spring (April–May) and early autumn (late September–October) are quieter and often very pleasant. The weather is mild, the forests are beautiful — particularly in October — and prices drop significantly. Many seasonal restaurants are still open in late September; some close in October.
Winter is quiet. The beach is empty, the Baltic is cold and dramatic, and most resort businesses are shut. A handful of bars and restaurants stay open year-round, and the town has enough permanent residents to maintain a low-key life, but if you're coming for the beach-and-terrace atmosphere, winter will disappoint.
The Sopot Festival (pop and rock concerts at the Forest Opera) takes place in late July or early August and is worth timing a trip around.