NOTE: It has been raining continuously for the past week, hopefully that means things will clear up over the next week. Also note that the temperature has been slowly creeping up and will average around between 16-25C in the upcoming week.
Here's an idea of what the forecast is from the 20th of May:
Saturday, May 20: High of 22C -- Mainly Sunny
Sunday, May 21: High of 23C -- Light Rain
Monday, May 22: High of 22C -- Chance of thundershowers
Tuesday, May 23: High of 17C -- Cloudy periods
What does that mean exactly?
A high of 23C with humidity could actually feel like 29C in Toronto.
A high of 23C also means that the low may be around 13-17 C.
What should you pack?
Sunglasses
Layers are key
Light spring attire
A spring coat and/or jumper(s)
You might consider something fit for rain just in case.
Another thing to keep in mind is that mornings and evenings tend to be chilly.