The Toronto International Trap & Skeet is open on Sundays all year round starting at 9am. They shoot trap, skeet and 5-stand.
It also seems to be open most Saturdays (9am) and perhaps Wednesdays in the summer. I would confirm the exact time for these days with the club if you plan to shoot other then on Sunday.
I am not a member but shoot trap there maybe 15 times a year usually on a Sunday. I just pay the $9/round instead of the $6/round that members pay. The italian sausage sandwich's at the snack bar are killer.