Assuming that one is able to visit an event as an observer like a ghost, without being able to be seen or heard or touched by the local / contemporary people, I would want to visit Dealey Plaza in Dallas on 22nd November 1963. I firmly believe that Lee Harvey Oswald acted alone in assassinating JFK, but it would be satisfying to be able to watch it actually happening, and have the smug feeling of certainty of knowing what had happened, rather than relying on ex-post-facto research and witness testimony. Of course in this scenario one would have the benefit of knowing in advance the best place from which to watch, and exactly when and where to watch, to be able to see it clearly.