Copyright registration is voluntary in the U.S. and in Canada (due to international copyright requirements). Each country however has a voluntary government registration system which provides you with certain advantages if you choose to register. In both countries, it is best to register upon creation of a work but you can register at any time and it is highly advisable you register before commencing a law suit as there are specific advantages if you do so.
One difference between the two registration systems is that you must deposit a copy of the work with the U.S. Copyright Office whereas the Canadian Copyright Office does not accept any deposits — you merely fill out the form and pay your fee. As such, many Canadians choose to register their work in Canada (for advantages in a copyright infringement suit) and also in the U.S. so a copy of the work is deposited and can be called upon should an infringement issue arise.