This might be a history question as much as a philosophy question but is there something profoundly distinct about the 20th and late 20th century that represents a distinct break from the past that is unlike any other break from the past in terms of its general significance? I honestly feel that is the case but then perhaps every century has felt that way.

No doubt many people feel, particularly around the turns of centuries, that something big/new/different is unfolding. But whether something big etc IS unfolding is probably only something that can be appreciated in retrospect, historically, long after the fact -- by historians, as you suggest. We tend to look back and see major shifts (cultural, intellectual, political etc) with the rise of the renaissance, and then the enlightenment -- whether that is just our tendency to carve things up into neat categories and narratives in retrospect, or in fact reflect "real changes", is a matter for the historians and philosophers of history to debate ... So to respond to your question more directly: first you must specify exactly the ways in whcih you think the 20th century marks a distinct break (cultural, intellectual, political, esp. technological i'm sure). Then you must meanignfully, on the basis of historical research, examine the kinds of distinct breaks earlier periods have displayed -- and then you must...

Without considering the arguments that there was ever a Jewish Holocaust can I be certain that such a thing happened just because I've read about it in my history books in school?

Why mention the Holocaust example specifically? Any worries about the "certainty" of historical knowledge would equally apply to every single piece of historical knowledge. Of course, what makes the Holocaust example stand out is that it does get challenged -- by people who have typically deeper agendas -- so perhaps what you should be asking is this: whenever you read about any historical event, and whenever you find people challenging conventional historical events, can you distinguish what is driven by "agenda" and what is driven by actual consideration of the available "facts"? (An excellent general book on the subject is the recent book "Voodoo Histories", which is a study of various conspiracy theories (including Holocaust denial and others), trying to articulate how/when people with agendas choose to selectively apply ordinary standards of reason and evidence ......) best, Andrew