All the Atlantic Provinces lie within the North Temperate Zone, which means that they have weather much like New England in the United States. Spring is damp, cool, and short, though it can get warm and muggy as it eases into summer.
Summer's compact high season runs from early July to early September. That's when the great majority of travelers take to the road, enjoying the bright, clear days and warm temperatures. The average high in the southern three provinces is in the upper 70s°F (around 25°C); in Newfoundland, it's more typically in the upper 60s°F (around 20°C). Nights can become cool, even approaching freezing, by late summer.
Be aware that there is no "typical" summer weather in Atlantic Canada. The only thing typical is change, and you're likely to experience balmy, sunny days as well as howling rainstorms -- quite possibly on the same day. Travelers who come here prepared for an occasional downpour, both psychologically and equipment-wise, tend to be happier than those who expect all blue skies. That's because the weather in all four provinces is to a large degree affected by the ocean. This means frequent fogs, especially on the Fundy Coast of New Brunswick, the Atlantic Coast of Nova Scotia, and Newfoundland's Avalon Peninsula. The ocean also offers an unobstructed corridor for high winds, particularly on Prince Edward Island and Newfoundland.
Note that the ocean does provide some benefits: Prince Edward Island's summer tends to linger into fall, thanks to the warm, moderating influence of the Gulf of St. Lawrence, and you'll rarely experience a sultry hot, humid day because of the natural air-conditioning action of the sea breezes.
Fall is a time of bright leaf colors but also rapidly cooling temperatures, especially at night, and much shorter daylight hours. Bring winter sweaters and a heavy coat.
Few travelers tackle the Maritimes in the dead of winter, as frequent blustery storms sweep in off the Atlantic. But if you're one of those hardy souls who might, be aware that snow or ice storms are a very real possibility at any time during winter, and they can blow in suddenly; if you're driving, make sure your car is equipped with good snow tires and special antifreeze windshield wash (you can get it from any gas station). And drive cautiously: Outside the major urban areas, most of this region's high-speed arteries are two-lane roads sans medians. Watch for drivers coming your way.
The national holidays in Canada are celebrated from the Atlantic to the Pacific to the Arctic oceans; for the traveler, this means all government offices and banks will be closed at these times. (Shops remain open on some but not all national holidays.) National holidays here include New Year's Day, Good Friday, Easter Monday, Victoria Day (the third Monday in May, always 1 week before Memorial Day in the United States), Canada Day (July 1; this is a biggie -- expect fireworks), Labour Day (first Monday in September, same as in the U.S.), Thanksgiving (mid-October; the same as Columbus Day weekend in the United States), Remembrance Day (November 11), Christmas Day (December 25), and Boxing Day (December 26).
Locally observed provincial holidays include a civic holiday (August 2) in Nova Scotia; New Brunswick Day (the first Monday in August); and several holidays in Newfoundland and Labrador, including St. George's Day (April 26), Discovery Day (the third Monday in June), and Orangeman's Day (July 12). Check out the proceedings.
Acadian pockets of New Brunswick, Nova Scotia, or Prince Edward Island also celebrate St. Jean Baptiste Day (June 23-24), which was actually pagan in origin (for the summer solstice, known in Europe as midsummer's night) but has since become associated with Catholic, Québecois, and Franco culture. Expect tons of Franco fun on this day.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.