September is the region's best kept secret. Warm water, empty roads, and every kitchen still open.
The short answer: go in September if you want the best combination of weather, price and access. Go in late May and June for icebergs off Newfoundland. Go in July for the fullest festival calendar and the highest prices. Go in October for the Cabot Trail in colour and accept that half the coastal kitchens have closed for the season.
May and June: icebergs and the first lobster
Iceberg season along Newfoundland's Iceberg Alley runs roughly late May through early July, with Twillingate and St. Anthony as the reliable viewing bases. Nova Scotia's spring lobster season is landing at the same time, which means the price at the wharf is at its lowest point of the year. Water is still too cold to swim anywhere.
July and August: peak everything
Prince Edward Island's beaches actually warm up, which is the one thing the Maritimes can claim over the rest of Atlantic Canada. Expect full accommodation, ferry reservations required for the Wood Islands and Digby crossings, and premium pricing. Book six weeks out minimum.
September: the month to book
The water is at its warmest of the year in early September, the roads empty after Labour Day, and the Annapolis Valley wineries are in harvest. This is the Tidal Bay window. Tidal Bay is Nova Scotia's own regulated appellation, introduced in 2012, and it is deliberately built as a low alcohol aromatic white to sit beside shellfish. Benjamin Bridge, Lightfoot & Wolfville, L'Acadie Vineyards and Domaine de Grand Pré are the producers to plan a day around.
October: Cabot Trail colour
Cape Breton's colour peaks in the second and third weeks of October and the Celtic Colours festival runs in that window. The trade is that many South Shore and Northumberland restaurants have already shut for the winter, and Newfoundland outport services thin out quickly.
What to buy and bring home
Nova Scotia sparkling wine travels well and is genuinely underpriced against Champagne. New Brunswick and PEI produce serious cheese and cured fish. Newfoundland's berry preserves, particularly partridgeberry and bakeapple, do not exist anywhere else in the country. Buy at the source rather than the airport, where the same jar carries a large multiple.