Title says it all.
Pearl Harbor? Berlin Wall? 9-11? Where we goin’? What would you like to see happen?
If you could travel in time to see a single event on a single day in a single place: where and when?
To only observe, you mean? The Wembley concert of Live Aid on July 13, 1985. Here’s what I’d have seen, not a flash in the pan among the bunch.
To influence a past event would be a whole other story.
Honestly of the big historical events I cannot think of a good one to view for the sake of viewing. So I will take Don Larson’s perfect game in 1956. Game 5 World Series over the Brooklyn Dodgers.
So that was October 8, 1956 at Yankee Stadium.
Probably Athens in 458 BC, for the first performance of Aeschylus’ Oresteia.
Or, roughly a millenium later, the first performance of A Midsummer Night’s Dream.
Modern era? Either a Hitler speech or Mayhem live in Sarpsborg, Norway, back in 1990, with Dead on vocals. 
Right now I think I’d say the signing of the Declaration of Independence. I’d really like to observe the whole process of creating the Constitution, but it was a multi-day affair.
If I’m indulging a profit motive, I’d go observe some unsolved mystery and bring the solution forward in time.
Philadelphia, 1787. Any day between May 25 and September 17, as long as I can hear James Madison debating at the Constitutional Convention.
I’d like to see Paganini perform, or Beethoven.
But I’d also like to see my folks get married, or my grandparents when they first met, or something more personal.
I’d like to see Vesuvius erupt and bury Pompeii.
Or just a random day and location during the Cretaceous Period, preferably on land. I’m certain that would be chock-full of Kodak moments.
V-J Day, On 14 August 1945 to celebrate with my countryman the end of WWII.
One of the happiest days in our history.
July 20, 1969, on the moon.
Or 5,200 years ago in the northern Italian Alps to see how Otzi the Iceman died.
No idea the year, but the crucifixion of Jesus Christ would be a one-day event that I’d like to see in person.
The Resurrection.
Yes. Just as Armstrong touches the surface, sneak up behind him and tap his shoulder. :eek:
Other than that, I’d love to be at the Premiere of Beethoven’s Ninth.
Beat me to it.
Does the fact that 2 out of 3 things that the OP mentions are things I actually witness make me old???
No, only if one of them Pearl Harbor would you be old.
Can we just see, or intervene? Because my answers are vastly different depending on that.
My own death. You didn’t say only the past.
A millennia later would be 500AD to nitpick. 
It’s a tough question. A lot of famous events are very violent, and I don’t think I’d enjoy the reality of people being perforated with sharp objects.
Most religious events are (I believe) fraudulent. Witnessing the fraud (or inaccuracies) would likely just make me more frustrated with superstition than I am already.
Most of history was conducted in languages I don’t understand.
Still, if I was given the opportunity, I’d come up with some answer, rather than saying, “Uhh no thanks.” Maybe, see a bit of pyramid construction.