Relate the "historical" events of the play that both preceded and are present during Claudius' reign.
Old Hamlet, we find out in an early scene, defeated Old Fortinbras (Fortinbras' father, King of Norway) in an ice-battle: and the gravedigger confirms that this defeat took place on the very day that Hamlet himself was born.
Young Fortinbras has recently sharked up his list of lawless followers to accompany him back to Denmark to reclaim the land his father lost - and the play opens with the guards talking about the sound of preparation for war: this war is anticipating a conflict with the Norweigans.
These events precede the play - and are crucial to its political plot.
However, its personal plot rather depends on a different series of events. Old Hamlet was murdered by Claudius, his brother, some months before the play itself began - Claudius then married Old Hamlet's widow (and Hamlet's mother) Gertrude. Their wedding happened quite close to Old Hamlet's funeral. And, just before the play begins (the night before, in fact) Old Hamlet's ghost appeared to the soldiers on the battlefield.
The chronology - i.e. precisely when the funeral and marriage happened, or precisely when Old Hamlet was murdered - or indeed, what day it is - is famously difficult to pin down. But as you can see, the events which go before the play and shape its course are those concerning the fathers: Old Hamlet and Old Fortinbras.