Ontario has a continental climate, with very defined season. In summer the temperature can reach 30°C, while in the winter it can drop to -25°C and snow is common and often very abundant. In spring and fall, the sun is often hidden behind rain clouds.
December to March is the ideal season for winter-sports enthusiasts (skiing, skating, etc.). Warm clothing is essential during this season (coat, scarf, hat, gloves, wool sweaters and boots). Toronto and the southwestern part of the province generally benefit from slightly milder conditions than the rest of southern Ontario.
Summer lasts from the end of May to the end of August and can be very hot. Bring along t-shirts, lightweight shirts and pants, shorts and sunglasses; a sweater or light jacket is a good idea for evenings. If you plan on doing any hiking, remember that temperatures are cooler in the forest.