In a short article on "The Big Idea" and his own preference for taking unusual paths in his writing, Fforde mentions that this particular dystopic post-apocalyptic fiction is set 700 years after the apocalpyse and explains why none of his characters discusses the "Something That Happened" in any detail:
So I chose my idea - Post Apocalyptic Dystopia - and then noted the well trodden path: The immediate aftermath of a global upheaval. The population in disarray, citizens fighting for survival in a new world order. Too obvious. How about seven hundred years afterwards, when the fall of mankind has no more relevance than the Dark Ages has to us today? I don't know about you, but I rarely talk about Edward III's scandalous claim to the French throne in 1337, but it's all people talked about then.
And of course, I immediately thought "Edward III had a scandalous claim to the French throne? Really? I mean, it was a claim through his mother, who was herself a scandal, but was Edward III's claim all that odd?"
I have managed to restrain myself from researching this issue. So far.