St-Valéry is a year-round destination, but it changes character sharply with the seasons. Late spring (May–June) is the best of the year: long days, mild weather, the bay alive with returning migrating birds, and the town quieter than in midsummer. September is the other sweet spot — warm enough for the steam railway and seal walks, with autumn light over the saltings.
July and August are the busiest months, when Parisian and Belgian families arrive en masse. The town is lively but cafés and restaurants need booking, and the upper-town lanes can feel crowded on weekends. Winter is quiet, often grey and windy, but it has its own appeal — the bay is at its most dramatic in stormy weather, and many of the heritage attractions close down or run a much reduced timetable.
Tides matter here more than weather. The tidal range in the Baie de Somme is among the largest in France, and the most striking sights — the seals on the sandbanks, the migratory birds feeding on the exposed mud — only happen at low water. Check the local tide tables (any tourist office or café has them posted) before planning a walk on the bay or a seal-watching trip.