The best times to visit Giethoorn are April–May and September–October. The village is photogenic in all seasons, but the shoulder months offer something close to the peace it is famous for — the crowds are manageable, the weather is mild, and the canals are navigable without the summer gridlock of boats and day-trippers. Spring brings tulip fields in the wider countryside and green vegetation along the canals; autumn turns the reeds and willows to gold.
Summer (June–August) is peak season, and peak means genuinely packed on weekends. The boat hire queues can be long, parking is scarce and expensive, and the canal system in the old village centre gets congested. The experience is still pleasant but noticeably more frantic. If you must visit in summer, arrive early (before 9am) on a weekday and aim to be on the water before the main wave of day-trippers arrives.
Winter is quiet, often too quiet — some cafes and boat hire companies close between November and March. But if you can find an open hire company, an out-of-season punt through misty canals has a quality entirely different from the summer version. The national park around the village is good for winter walks, and the bird hides are worth visiting for wintering wildfowl.