Toronto has a classic four seasons climate, with warm and humid summers, and cold winters where snowfall isn’t uncommon. The temperatures in summer rarely rise so high that it’s uncomfortable to be outside for most people, so it’s really the ideal season to visit. Winters definitely do get cold and it can go weeks at a time without rising above freezing.
Spring and autumn are usually quite nice, although it usually isn’t until May that it’ll be warm enough to do without a jacket or sweater during the day, so if you are planning on doing outdoor things then May through September are the best months.
The city gets mild and consistent rainfall throughout the year, with most of it being snow during winter. There’s no dry months or unusually wet months, so prepare for a bit of rain and cross your fingers.