Your Visa has a validity. For instance your visa will state valid till August 2012 or some date will be there. So as long as a) you have a visa for Canada and b) it has a validity date, you can come to canada. Suppose my course starts in January 2011 but I decide to come to canada in December 2010 it doesn't matter because my visa is valid till March 2012 because that's when my course will be over.
I am stating an example - but this is how it works.
hope it helped!