It's hard to say. I mean, it's difficult to narrow it down to any one event. If you take a look at one of the websites such as:
http://www.toronto.com/
You can find out what is planned. It's a pretty busy site, but if you poke through it you can find something. You may be in time for the Beaches Jazz Festival.
- Luminato starts this week, but will be over by the 20th.
- the Toronto Jazz festival will be on (not the Beaches which is bigger)
- the Big Festival is a multi location Street fair, it will be on
- Harbourfront will have the a French culture festival on the 21s (Franco Fete)
- oh yes, you will be here during Pride Week. It's big. All the usual festivities with the Dyke march & Pride Parade on the 28 & 29th.
- Dragon Boat festival on the 21st
- Ribfest starting on the 27th
- Portuguese Film Festival starting on the 26th
- Sensoria at Yonge dundas Square (near the Eaton Centre) an international French culture festival on June 20 - 22
- outdoor cinema also at Dundas square, every Tuesday night starting on the 24th (Dundas Square is surrounded by huge TV screens on all the surrounding buildings see:
http://www.ydsquare.ca/ )
Dundas Square:
Another website to try is Eye magazine:
http://www.eyeweekly.com/
it is less corporate than the above site, so offers a parallax view on happenings, and is also edgier/more interesting.
Hope this helps!