Map from Google - Montreal

In Montreal, the main city of Québec, the climate is continental, with very cold winters and warm summers. Although the city is located on the 45th parallel, therefore halfway between the Pole and the Equator, in winter it's freezing because the North American land mass cools down a lot, and outbreaks of polar air masses are frequent.
Here are the average temperatures (of the airport of Saint-Hubert, a bit out of the city).
Average temperatures - Montreal
MontrealJanFebMarAprMayJunJulAugSepOctNovDec
MontrealJFMAMJJASOND
Min (°C)-15-13-70712151493-3-10
Max (°C)-6-32111924262521136-1
Min (°F)5919324554595748372714
Max (°F)212736526675797770554330

Precipitation amounts to about 1,000 millimetres (40 inches) per year, and is well distributed over the seasons: in fact, weather fronts pass in all the months, due to continual clashes between masses of warm and cold air, and in the only season in which these clashes are less frequent, ie in summer, thunderstorms can develop in the afternoon, adding more rain, so much so that summer is as rainy as autumn, at least in quantity. During the long winter months, from mid-November to early April, precipitation mostly occurs as snow. Snow normally covers the ground from late November to mid-March. In a year, more than two metres (6.5 feet) of snow fall; occasionally a snowfall can be seen already in October, and in spring, still in early May. In the winter of 2007/2008, a total of 436 cm (14.3 ft) of snow fell.
Here is the average precipitation.
Average precipitation - Montreal
MontrealJanFebMarAprMayJunJulAugSepOctNovDecYear
MontrealJFMAMJJASONDY
Prec. (mm)75607085808595908585105901005
Prec. (in)32.42.83.33.13.33.73.53.33.34.13.539.6
Days141112111313121312131415153

Winter in Montreal runs from mid-November to mid-March, and is very cold, so that the average temperature, about -10 °C (14 °F) in January, is similar to that of Moscow, Russia. However, at least here the days are not as short as in Nordic countries, because as mentioned the latitude is not very high.
Waves of freezing cold are accompanied by the Blizzard, the north wind blowing in gusts, or by the Barber, the cold and wet wind able to freeze beard and hair, which blows from the north-east, ie from the Gulf of St. Lawrence.
Given the constant weather instability, the temperature may exceed freezing (0 °C or 32 °F) also in winter, so that snow can melt and it can rain. On the other hand, intense cold waves, with lows around -25/-30 °C (-13/-22 °F), occur practically every year.
Snowfalls are frequent, so much so that it snows for several days per month; occasionally there can be real snow storms, like that of March 4, 1971, with 43 cm (17 in) of snow in 24 hours, or that of December 27, 2012, with 45 cm (17.7 in) in 24 hours.

Spring, from mid-March to May, is an unstable season, characterized by temperature swings; it is initially cold in March, with snowfalls and frosts still frequent, then it gradually advances, between the first warm days and the return of cold weather, and it's still normally cold in April, while it becomes mild in May, when also thunderstorms begin to develop in the afternoon.

Summer, from June to August, is quite warm, and when the temperature is higher than usual it is also sultry, due to the contribution of the rivers, Saint Lawrence and Ottawa, which widen near the city, creating lakes that provide additional moisture. It is said, a little ironically, that in Montreal there are only two seasons: winter and the month of July: obviously this is an exaggeration, but there is some truth, in the sense that July stands out as the warmest and sunniest month; August is not really so different, but it's already a bit cooler and less sunny. However, in summer there's no shortage of cool and cloudy days, while on fine days, thunderstorms can erupt in the afternoon. There can be periods in which the weather is hot and humid, with lows around 20/22 °C (68/72 °F) and highs around 32/33 °C (90/91 °F), which typically only last a few days; more rarely it can even get hotter, as happened in July 2011, when the temperature reached 36 °C (97 °F).

Autumn, from September to mid-November, is initially pleasant, with many sunny days, in which the temperature exceeds 25 °C (77 °F): this kind of weather, called Indian summer, occurs in September, but sometimes also in the first half of October. As the weeks pass, however, the temperature decreases rapidly, bad weather becomes more frequent, and the first cold days occur.

The amount of sunshine in Montréal is good in summer, and it is acceptable in May and September; in the other months it is mediocre, and is scarce in winter, especially in November and December, when the sun is very rarely seen.

Montréal

When to go

The best time to visit Montréal goes from mid-May to late September. In midsummer, especially in July and early August, there can be hot and muggy days, so this period can be excluded if you cannot stand the heat; in May and early June it can still be a bit cold. However, you have to take into account the possibility of rain and changes in temperature.