Local weather: what to wear. (Montréal)

Greetings! I'll be leaving this Sunday and I'll be spending a week in Montreal.
I'll be coming from a very, hot summer (our present weather system has been named "Lucifer"!!!!), about 38/42 degree celsius.
I checked the weather report for Montreal, however I have no ideal what to wear: are sandals ok? Sleeveless? I detest packing my luggage with useless stuff! I would really appreciate some feedbacks.
Thank you.
Vanda