So, “event film” could mean any one of the following:
A) The film is about a significant event in history, much like how Dunkirk was a film about the significant event of the Dunkirk evacuation.
2) The release of the film is going to be a significant event in itself, where everyone remembers the anticipation and finally getting to see the great phenomenon that is Nolan filmmaking for generations to come.
D) Christopher Nolan and Hoyte Van Hoytema were hired to shoot someone’s wedding in IMAX, and it’s going to be epic.
I think 2) is a guarantee, regardless of what Warner Brothers means by “event film”.
DoP Hoyte van Hoytema is planning to re-team with Christopher Nolan on his upcoming production scheduled to begin filming this June. The Warner Bros. feature is described as a romantic thriller, NORTH BY NORTHWEST in tone meets INCEPTION