Ventspils is at its most pleasant from June to August, when the daylight is long (sunset after 22:00 in mid-June), the Baltic warms enough for swimming, and the outdoor cafe-and-beach life is in full swing. The town's biggest annual event, the Ventspils City Festival, takes over the first weekend of August with concerts, parades, and fireworks.
May and September are quieter, cool but pleasant, and good months for cycling without the summer crowds. Winter is dark and cold (-5 to -15°C from December to February) and most of the beach-related infrastructure shuts down — but the castle, the museums, and the aquapark are open year-round.