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Generally speaking people recommend no more than 6 weeks in advance. Keep in mind that you can't start working until your classes start. So if you come early, you need to bring additional money to support yourself since you wont' be able to work during this time (and Canada is expensive).
 
Bro my sister lives there , I want to go 3 months before September , In June , I guess... Without considering expenses, will Canadian government allow me to come to Canada 3 month before my classes start?
 
Bro my sister lives there , I want to go 3 months before September , In June , I guess... Without considering expenses, will Canadian government allow me to come to Canada 3 month before my classes start?

Nope, you can only go a month before the 1st day of your class.