I found the following information on Algonquin Park's website.
In Algonquin, blackflies are usually out by mid-May (depending on the weather in any given year) and are usually around until late June. The worst time of day for blackflies tends to be the last 2 or 3 hours of sunlight.
Mosquitoes are also abundant, beginning in mid to late-May and usually last longer than blackflies (into July). Mosquitoes are most often a problem in cooler, shady parts of the forest, as well as in the evening, and into the first couple of hours of darkness. They usually become less of a problem through the night (although they do not disappear entirely).
How to Prepare Yourself
Eat lots of garlic for at least a week before you come — use it in every meal. Your friends may stay away from you but so will the blackflies! Evidence exists that something in garlic acts as a repellent.
So, it helps to be a garlic freak if you want to go camping. It will help you keep away blackflies and very probably mosquitoes too!