There should be an expiration date on your visa that tells you of the final day you can travel in Canada, regardless of the days you requested on your application. If you're within the window of validity, you should be good to go.
Your visa is typically good for six months – flights get delayed, cancelled, shifted around and governments recognize that. Like all countries it's entirely dependent on the border guard to issue a final date you can stay in the country, regardless of your visa expiration. They can give you until the final day of your visa, or the day after your departure ticket. It's pretty much up to them.
Finally and most importantly, enjoy your stay in Canada this summer!