Works in the public domain are not protected by copyright, and they can enter the public domain for several reasons including expiration of copyright, the creator having waived copyright, or the work not being eligible for copyright protection.
Public domain works can be used without permission from the creator. It is not legally necessary to cite these works, but it is a best practice to do so, particularly from the perspective of academic integrity.
Copyright expiration in Canada: The Copyright Act states that copyright in a work exists for the life of the author/creator, plus the remainder of the year after death, plus fifty years. However, when original content is added to a public domain work (e.g. a new translation or annotations), that content is protected by copyright.
No attribution statement is required, but you can add one if you like.
This image is in the public domain because the creator waived her rights and shared it under a CC0 license:
Optional attribution: Photo by Paula May on Unsplash
This image is in the public domain because the creator's copyright has expired:
Optional attribution: Lear, E. (1835). Ring-necked parakeet (psittacus torquatus), [Image]. McGill University Library. Internet Archive.