When a work is no longer protected by copyright, it becomes part of the "public domain". Once a work is in the public domain, none of the limitations or restrictions of copyright apply to the work. Works in the public domain can be used freely.
In Canada, generally the term of copyright lasts for "the life of the author, the remainder of the calendar year in which the author dies, and a period of fifty years following the end of that calendar year" (Copyright Act, s. 6). Upon the expiry of the term of copyright, the work enters the public domain.
UBC Resources on Public Domain
Canadian Public Domain Flowchart