act i scene 1

We get this in Act one scene one when the guards and Horatio see the ghost of the dead king of Denmark,

Horatio:

What art thou that usurp’st this time of night

Together with that fair and warlike form

In which the majesty of buried Denmark

Did sometimes march? By heaven, I charge thee, speak.

 This knowledge is crucial to the story because it sets the stage for Hamlet's anxieties and eventual revenge.

we're informed very early (in the very first scene when the guard see the dead king's ghost)

we must be informed early because it's what sets the story ,it's a crucial element of it, the story is based on what the ghost says and hamlet's desire to revenge his father's death