Dunkirk can refer to two related things:

  • The historical event: the Dunkirk evacuation (Operation Dynamo) took place during World War II from 26 May to 4 June 1940.
  • The movie: Christopher Nolan’s film “Dunkirk” was released in cinemas on 21 July 2017 in the US and UK, after premiering in London on 13 July 2017.

Since your question is short (“when was dunkirk”), people usually mean either the 1940 evacuation or the 2017 film, so both dates are often given in answers.