Toronto is a most wonderful city to visit any time of year.  Here are a few tips on how to deal with the weather and when to go.
If you bring the proper attire, then any time is a good time to visit the incredible city of Toronto.

Weather Forecasts and Averages

If you've already picked the dates for your trip, you can find out what to expect:

However, there's no way to know exactly what the weather will be.  You should always pack an umbrella in case of rain and some extra warm clothes in case the weather turns cold.

Seasonal Tips

Toronto enjoys the full experience of all four seasons, so you'll get a different feel to the city depending on when you visit.

Winter 

By early December, the first signs of winter weather can appear.   The chances of experiencing a "white Christmas" in Toronto is about 35%. It can get cold in December and January and even some Februarys are cold and windy, but it's rare you have to jump over snowbanks if you are in the city. However, if you have a good winter jacket, winter boots, a good Canadian toque (that's a wool cap for you outsiders), scarf, mitts or gloves you will be prepared for the weather.  An extra sweater is recommended which you can wear under your coat if it is particularily cold. You can always leave the sweater behind if it is a mild day.  Layering can be important to keep a harsh winter chill from your bones.  You can always take clothing off if you are too hot, but you''ll freeze if you do not have enough layers on! (Probably throwing in a pair of long underwear wouldn't hurt either!!)

In comparison to some of the very remote areas of Ontario, where winter usually starts in October and generally ends in April with tons of snow and many, many freezing cold days, Toronto's winters are not that cold or that harsh. The average temperature in Toronto in January is -1.3°C (30°F). However daily temperatures can range anywhere from -22°C (-8°F) to +16°C (61°F). There are usually a few cold snaps every winter, and usually a couple of pretty good snow storms. There are also many very mild days as well., often a "January thaw" will set in the CIty.

Toronto does not get the severe weather that many other cities in such close proximity to large bodies of water can get. In conclusion, with regard to Toronto's winters, as long as you are dressed for the elements you will be able to deal with them.

As far as getting around Toronto, if you are leery about driving in snowy conditions, you can usually rely on public transit or even taxis.  If you are in downtown Toronto, it may be possible to get from one place to another without even going out into the cold by following the PATH, an underground walkway linking 27 kilometres of shopping, services, and entertainment.   The city streets are fairly quiet during the winter season, but don't be fooled.  Clubs are hopping, world class live theatre is available, and the Leafs (hockey), Raptors (basketball) and Rock (lacrosse) are in full season.  

Spring

By mid-March, spring is in the air and the transition to more outdoor society can be seen.   Baseball season arrives in early April and the Rogers Centre (formerly SkyDome) guarantees a good climate for ball fans.  By May, the temperature is very comfortable and is typically a light jacket season.  May is typically the start of patio season at many restaurants in Toronto.

Summer

Summer months in Toronto are from June to September.

The summers in Toronto can be hot and humid with a few smog days thrown in. Summer temps in Toronto can range anywhere between 20°C to 35°C (68°F to 95°F). It's perfect weather for going to all the outdoor activities and events around Toronto including Centre Island, Canada's Wonderland, the Toronto Zoo, the Beaches Jazz Festival, Taste of the Danforth or driving north of the city to experience the wonders of Muskoka and other lakes. Pack mostly clothing for fairly hot weather, but be sure to bring a light jacket and sweater for evenings (and for shopping in malls and dining out, as the air conditioning can make it very cool.)

Anyone visiting Toronto in the summer months, especially July and August, should come prepared in case it rains.  Sometimes, seemingly out of the blue, it gets very dark and the weather can turn quite unpleasant followed by brief downpour. Nonetheless, umbrellas and water proof hoods are always handy.

Note that if a person has asthma or any type of respiratory condition they should watch the forecast closely for smog warnings for Toronto, as it can pose a health risk if you are prone to those conditions. Visitors would be more comfortable in air conditioned accommondations while visiting in the summer months. 

Fall

September and early October are fantastic weather months, often at very warm temperatures and little rain.  Fall can be felt in the air by mid-October and the leaves start to turn.  Drives north of Toronto can reveal dramatic leaf colours -- a detailed fall colour report tracks progress across the province.  Speaking of leafs, the Toronto Maple Leafs start up yet another quest for the Holy Grail (aka the Stanley Cup) in the fall, and the Raptors hit the floor as well.  November is often a cool and rainy month in Toronto.