I could have told you it was a weekday, because I was at work, but I'll take your word for it being specifically a Tuesday. I think 9/11 is a bit of a cheat, though, because it's world-wide referenced by it's date. How many people, a couple of generations on, would remember the precise dates of, say, JFK's assassination or the moon landing? (I know the second, but not the first, and I was a history major!)
> How many people, a couple of generations on, would remember the precise dates
The people who witnessed Pompeii in person probably remembers it pretty well. This is why I trust his recollection, even if Pliny wrote it down decades later.
It's also not unreasonable to assume he talked about it several times between the event and when it was written down, so that it was easy to remember the details.