When to come — and when to absolutely avoid
By season
Quietest: Mid-January to mid-March (excluding February school holidays) and the first three weeks of November — Normandy in the off-season can be cold and damp, with low sea mists wreathing the abbey, but the interiors are wonderfully empty and the first Sunday of every month from October to March is also free. Busiest: Mid-July through August, Easter weekend, French school holidays, every weekend from June to September, and the entire week between Christmas and New Year. Saturdays from April to October are always heaviest.
By day of the week
Tuesday and Wednesday mornings are calmest. Saturday is the worst day — French families on weekend trips overlap with coach groups from Paris and Saint-Malo. Avoid Sunday morning if the abbey church is a priority: Mass is held there and that part of the church is reserved for worshippers.
By time of day
The first hour after opening at 09:00 (or 09:30 in winter) is the calmest of the entire day — coach groups typically arrive between 10:30 and 11:00, peak from 11:30 to 15:30, and thin out noticeably after 16:30 when day-trippers head for their evening transfers. Late afternoon (16:00 onwards) and early winter mornings are the quietest single windows. If you can stay on the island overnight (a few inns on the Grande Rue), you'll have the village to yourself between 19:00 and 08:00 — one of the most magical experiences in France.
The tides
Twice a month around the new and full moons, the bay experiences grandes marées ("great tides") when the water can rise up to 14 metres and briefly isolate the island again — a spectacle the medieval pilgrims would have known well. Dates are published a year in advance by the Service Hydrographique de la Marine; aim for a coefficient above 100 for the full effect.