1717 – The Ministry of Magic declares that the Cruciatus, Imperius and Avada Kedavra Curses are classified as Unforgivable. Severe penalties are laid down for the use of these Dark magic spells (TBB).
Three Dark curses are defined as Unforgivable
Certainty Stated in canon
Location in Canon TBB: The Tales of Beedle the Bard (book)
Type of Event Wizarding world