If you could go back in time and see a legendary or not-so-legendary concert what would it be and why?
try to avoid Woodstock and Live Aid
I’d like to see Queen in their heyday. I only got to see them on their last US Tour and it wasn’t working as well as it had, I feel certain.
Heck, there’s not much I haven’t seen in my era. But going further back I’d like to see Sinatra, Elvis, and maybe the Beatles. Just for the joy of it all. Maybe the Ink Spots.
The Beatles at the Cavern.
I’ll second silenus.
I’d also like a chance to see Nirvana again…didn’t realize the last time would literally be the last time. 
Since there’s no rule about it not being a show you actually went to, I would set the Wayback machine for Shea Stadium circa 1985 for David Johansen, The Clash and The Who.
Best. Show. Ever.
I also would have liked to have seen the Concert for Bangladesh in person.
Any KISS show circa 1978.
January 29, 1967. The Saville Theatre, London.
Opening act:The Jimi Hendrix Experience, followed by The Who. Also noteworthy as it was the first time “A Quick One While He’s Away” was performed live.
I’d love to have watched Metallica back in the “Master of Puppets”/"…And Justice For All" days. I’d even settle for a Black Album tour show, but anything after that… meh.
I’ll also second seeing a KISS show from back in their heyday.
The Beatles in The Cavern as well.
The Monterrey Pop Festival with The Who and Hendrix.
The Dead during the acid test days.
Dylan in a coffee house in NYC before going big.
Cream.
The 1970 Isle of Wight Festival (performer list from Wiki.
The Free concert in Hyde park, 1969, which included the Stones and the debut of King Crimson.
California Jam.
The Beatles, at Shea, or on the roof of Apple records, or their “last” show at Candlestick, or maybe the Sullivan show.
Sarahfeena, I would love to have seen Nirvana!
Dylan going electric at the Newport Festival in '65 (I was four at the time, and had other plans).
The Ramones at CBGB, any one of 'em.
The Doors when Morrison made the performance that got him arrested for indecent exposure (just for the hell of it).
The Concert for Bangladesh.
The full bill the night Buddy Holly (and Ritchie Valenz and the Big Bopper) died.
The Sex Pistols, any night of their first American Tour (you know, the one before the Filthy Lucre tour).
Elvis, any of the Sullivan performances.
The Stones at Altamont, just because.
Cheap Trick, at Budokan (you know which night I mean).
The Allman Brothers, at the Fillmore East.
The Quarrymen, the performance just before Lennon met McCartney.
Frampton, you know, the one where he came alive. Just because I can’t resist, with this time machine and all.
I was there. 
Really? That is so cool. Any details to share? (Or are you joshing?–sorry, this would be VERY cool. Don’t toy with me.)
Nope. It was just one of those nights…magic. It’s true that the whole album wasn’t recorded at Winterland, but it doesn’t really matter. Concerts at Winterland–in those days in general, really–were different than today. Just a different level of involvement with what was going on up onstage. (possibly due no cellphones, or maybe more drugs being consumed?)
Right before Frampton and Co. came on, I tossed a “glow stick” from my seat on the balcony. It bounced of someone’s head (sorry!) and ended up in front of the drumkit. Sadly, a roady came out and picked it up before Peter walked onstage. Only a few other times have I witnessed an audience erupt in such applause and, I have to say it, love, for a performer.
BTW, they just issued a DVD of “Frampton Comes Alive 2”, from a 1995 performance. Great stuff.
Sorry for the hijack…we now return you to the original thread…
Ah, Winterland. I saw Santana there one night. Truly magic.
OK, time for a hijak on what concerts us old farts were at. I was at Watkins Glenn in '73 (The Band, The Allman Brothers, and The Grateful Dead; an all “The” bill). I was at the first Galstonbury Concert in 70 or 71 (Traffic, Melanie, and others I forget). That same summer I saw a concert in Hyde Park with Traffic (again) and Grand Funk Railroad.
[continuing the hijack] I’m on most of the Springsteen Live album, at least the tracks that were recorded on the West Coast. I was at the ARMS Benefit in LA (Clapton, Winwood, Cocker, Beck, Page, Watts, Wyman) and the Steve Goodman Leukemia Benefit (Ronstadt, Nelson, Newman, Browne, Mull, half the Eagles)[/end hijack]
I would pick any Queen show,preferably one of the huge Argentine shows or the free concert in Hyde Park or maybe one of the real early shows at The Rainbow.
Does it have to be a rock concert?
I’d want to be at Aeolian Hall in New York City (42nd St., across from the library) on February 12th, 1924. The concert was called “An Experiment in Modern Music”. It was the world premiere of Rhapsody in Blue, with George Gershwin on piano.
Another vote for Isle of Wight, 1970.
So-called ‘prog rock’ was all the rage when I was a teenager, but I went to see very few live shows. Given the OP’s time machine, I’d like to go back and see bands like Genesis and Yes in their glory years, when they were putting on truly gob-smack-tacular live shows.