Eastbound day flight options from North America to Europe are limited, and from Toronto you will almost certainly need at least one and possibly two nights of extra lodging to make it work.
The only European destination with day flights is London. You can fly there from Toronto (on AC), Boston (AA), New York (AA, BA, UA), Chicago (AA), or Washington (UA). That's it as far as I know.
The reason you'll need extra lodging is that given the timing of the flights, they will either leave too early for you to make it from Toronto the same morning, or they arrive in London too late for you to make an onward connection to Athens the same night. If the timing is unlucky you may end up having to spend both one extra night in your North American departure point and one extra night in London.
(As a sidenote, this is a hint of why eastbound transatlantic daytime flight options are limited -- connections are hard at either or both ends, which makes them harder economically for the airline.)