Quimper is at its best in summer, particularly July when the Festival de Cornouaille transforms the city for a week of Celtic music and dance. The weather in July and August is generally good — not as sunny as Mediterranean France, but pleasantly warm with sea breezes from the Atlantic coast 20 kilometres away.
Spring (April to June) and early autumn (September) are quieter and often have good weather, with fewer crowds in the old town and at the museums. Winter is mild by French standards but grey and wet — best avoided unless you specifically want the city to yourself.
The Saturday market on Place Saint-Corentin is worth building a visit around regardless of season.