I'm thinking late April or May before the long weekend. IMO after May 24th, you are competing with everything else in our lives including kids out of school, cottages, other car meets, I'm thinking the sooner the better. April we have the risk of rain but things usually turn out nicely on or around Mother's day it seems. When I get the time, I'll try and get the "probable rain dates by statistics" and nail a weekend that is likely going to be sunny.
Here is some trivia.....the organizers of the annual Toronto Indy Race (back when it was called that) did research before the series started regarding probable rain dates in Toronto during the summer months. If my memory serves me, the race is usually the second week in July. Sure enough, on race day (Sunday) every year they have come to Toronto, they have had an overwhelming success with sunshine, in fact, I can't ever remember the race in the rain. Qualifying yes, but not on race day. True story!