I can't predict the weather three months out. However, I can point you at [average temperatures][1] for Toronto in September. It looks like it could be hit-or-miss, depending on your tolerance for cold, and that the earlier in the month, the better your chances, especially during the warmest part of the day. Note also the average rainfall of 2.9" for the month. 

I can also link to [this chart][2] from the Canadian seasonal prediction system, which appears to show a broad general trend toward temperatures above normal during this time. 


  [1]: https://weather.com/weather/monthly/l/CAXX0504:1:CA
  [2]: https://weather.gc.ca/saisons/image_e.html?img=s234pfe1t_cal&bc=prob