Generally about 1 month before classes start. If you want to enter earlier, you'll need to be able to show proof that you have the additional funds required to support yourself in Canada without working.
Keep in mind that you cannot start working until your classes begin.
My classes will also begin in Sept. I arrived in Canada about a week ago, I received permits for myself and accompanying spouse and child without being asked any questions and so waa both of my friends that entered Canada separately this week.