Toronto, where it's located

In Toronto, Canada's largest city and the capital of Ontario, the climate is continental, with cold winters (when there are many periods below freezing) and warm summers. Although the city is located in the south of the country, winter is cold because the North American continent cools down a lot, and outbreaks of polar air masses are frequent. However, given the exposure to different types of air masses, jumps in temperature are frequent.
Here are the average temperatures at the Toronto Pearson Airport, in the south-west of the city. The neighbourhoods overlooking the shores of the lake are a bit milder.
Toronto - Average temperatures
Min (°C)-9-9-5271316151150-6
Max (°C)-2051219242726221481
Min (°F)161623364555615952413221
Max (°F)283241546675817972574634

Precipitation amounts to about 800 millimeters (31.5 inches) per year, and it's well distributed over the seasons. In fact, disturbances can affect this area in all the months because of the continual clash between masses of warm and cold air, while in summer, when they are less frequent, afternoon thunderstorms can occur. So it's no coincidence that August is the rainiest month, at least as regards to the amount. During the long winter months, from mid-November to March, precipitation occurs mainly in the form of snow. In an average year, about one meter and a half (5 feet) of snow fall; sometimes a snowfall can be seen already in October, and in spring, still in early May.
Here is the average precipitation.
Toronto - Average precipitation

Fog is fairly common at night and in the early morning, all year round, especially in rural areas around the city.

Toronto skyline

Winter in Toronto runs from late November to mid-March and is cold: the average temperature, about -5.5 °C (22 °F) in January, is similar to that of northern European cities like Oslo. However, at least here the days are not as short as in the Nordic countries.
Cold waves are accompanied by the blizzard, the north wind blowing in gusts.
Given the constant instability of the weather, the temperature in winter can exceed freezing (0 °C or 32 °F): in these cases, snow can melt and it can rain. On the other hand, waves of intense cold, with peaks of -20/-25 °C (-4/-13 °F), occur practically every year.
Snowfalls are frequent, so much so that it snows for several days per month; occasionally, real snow storms may occur. Sometimes the phenomenon called "ice storm" (or glaze ice) may occur, that is, rain falling on frozen ground, freezing everything, roads, trees, light poles, cars etc.

Spring, from mid-March to May, is an unstable season, characterized by changes in temperature: initially cold in March, with snowfalls and frosts still quite frequent, it advances gradually, with the first warm days but also the return of cold weather, and it is still on average cold in April, when it can still snow in the first half of the month, while it becomes mild in May, when also afternoon thunderstorms start to erupt during sunny days.

Summer, from June to August, is warm, with highs usually about 26/27 °C (79/81 °F); on hot days, when the temperature can reach or even exceed 30 °C (86 °F), it can also be muggy because of the humidity brought by Lake Ontario. However, in summer there are also cool and cloudy days, while in days of fine weather, which are cooled by the breeze from the lake, thunderstorms can erupt in the afternoon. Sometimes even heat waves may occur, with highs around 34/37 °C (93/99 °F), generally of short duration.

Autumn, from September to mid-November, is initially pleasant, with many sunny days, when the temperature exceeds 25 °C (77 °F): these periods, called Indian summer, occur in September, but sometimes also in October. As weeks pass, however, the temperature decreases rapidly, low pressure areas become more frequent and the first cold days occur.

The amount of sunshine in Toronto is all in all good in late spring and summer; it is not high in the other months, and it's quite low in winter, when the sun is rarely seen.
Toronto - Sunshine

Lake Ontario is very cold in winter, when it's close to the freezing point, while in summer it's still cool, but it becomes almost acceptable for swimming, at least for those who do not suffer from the cold, especially in August, when it reaches 21 °C (70 °F).
Here are the average lake temperatures.
Toronto - Lake temperature
Temp (°C)32237131921191495
Temp (°F)373636374555667066574841

Best Time

The best time to visit Toronto runs from mid-May to late September. In the height of summer, ie in July and August, there may be hot and muggy days, so this period can be excluded if you cannot stand the heat; in May (and sometimes in early June), it can still be a bit cold. However, you always have to take into account the possibility of rain and changes in temperature.