The go-to place for this sort of question is either the Web site of the country in question (here Canada) or, for U.S. citizens, the Country Information page of the U.S. State Department's site. Clicking on Canada there says, right under Quick Facts, "PASSPORT VALIDITY: Must be valid at time of entry."

(For smaller countries that aren't as easy to zero in on as Canada in the small map of the world on that page, you can also enter their name in a box. Start typing and it will show you a list of all the places that match what you've typed up to that point. It also tells you what country your mouse pointer is over when it's in the map area.)