THis is a meta question, in reality, I'm not asking something about a specific historical event, but about the historical method. So i choose to ask historians about!
I imagine that sciences or fields of study select their subjetcs of study. A economist will study the exchange between the people, and will only matter about a natural fact when there is some economical consequence.
So, what is the "subject" of study of the historian? In a cartesian way of thinking, everything that happened before is important to what happened after. But the historian don't analyses the mood of a historical person in the day, if he quarreled with his wife, how was the weather, only in very specific circunstances.
And I believe too that a science/field of study, when empirical, is based in trying to form relations of causality (am I being positivist?), so what the relation of causality that the historian searches?
I know that the answer of my question will depend on the approach of the historian, but I am willing to know all.
I think E.H. Carr said it best when he said that history only becomes fact when someone decides it should be. (I'm on mobile right now so I'm just paraphrasing from my recollection of the reading. E.H. Carr What is History?) The creation of history is a subjective process based on the whims of whoever is working to create an historical "fact" and it ultimately becomes so when people accept it as so.
The criterion for what makes "legitimate" historical facts have, of course, changed over time as social and intellectual philosophy have changed. I think it goes without saying that this criterion has expanded over the centuries as more and more scholars and individuals fought for the legitimacy, rights and importance of groups and topics which used to be marginalized in historical study.
But, it's not just the decision of fact that can create historical fact, but also the presentation of said facts. An agreed upon set of facts can be presented in different narratives which influence our understanding of those facts. These narratives can be influenced by politics, society, culture, etc. Hayden White's The Content of the Form is still probably the classic on this particular issue.
This is just a quick set of thoughts (because I am on mobile and at work at the moment. Shhhhhhhh.), but I hope it gets some conversation going.
Not to discourage other answers, but you may be interested in this previous question asking what criteria are used to establish a historical fact. There are several very good discussions in that thread.
Additionally, you may be interested in the historiography section of our FAQ which is primarily concerned with question about how historians critically examine sources.