If you had access to a time machine, what concerts would you go back to see?

You are certainly not alone in 2020 being the first no-concert year since, for me, 1969. Tool and LP (Laura Pergolizzi) were on my schedule, but luckily I have seen them before.
As to concerts I wish I had seen.
The Allman Brothers "Live at the Filmore East"
Woodstock (without the mud)
Adele at Royal Albert Hall
The BBC Concerts from 1970 - 71 by Elton John, Neil Young, Cat Stevens and James Taylors. 
Any concert by Amy Winehouse.

@artemus_5,  I forgot about a couple of good ZZ Top stories:

1) At the 2003 SXSW Music Festival, I attended a Yardbirds show at The Austin Music Hall. I sat in the balcony, and a short time later Billy Gibbons sat down beside me. He was accompanied by two gorgeous women wearing tuxedos. It was hard to tell if they were "friends" or "bodyguards" (they were acting very serious). The Yardbirds show featured multiple special guest guitarists, including: Skunk Baxter, Joe Satriani, Slash & Steve Vai.  I asked Billy if he would be participating and he said "no, I wasn't asked." I laughed and said that if he showed up backstage, I'm sure he would get an instant invitation.

Ironically, the following year this venue limited their balcony to VIP's only. Even funnier, I told a friend of mine and her friend the whole story. What I didn't know was the friend of the friend had dated Skunk back in the day, and he didn't let her know that he was in town for this gig. She was not pleased about this......Oops, sorry Skunk!

2) My 5th concert was ZZ Top in the fall of 1973. "Tres Hombres" had not taken off yet and they were getting zero airplay in Atlanta at that time. A high school friend had spent the summer in Texas and he came back raving about the band; so, when the gig was scheduled he convinced a bunch of us to go. Tickets were only $5.00, so we figured we had nothing to lose. ZZ Top played their @sses off. While the crowd was small, they were beyond enthusiastic. Everyone was dancing (nobody sitting down) and I recall copious amounts of ganja being passed around. We had a great time and were suitably impressed.

So great to see someone other than me mentioning The Blue Nile in these fora. I had the great pleasure of seeing them at a very small club in Western Massachusetts in 1996, and the same thing happened: They played brilliantly, connected with the audience, and were brought back after their encore by an audience that had had a spiritual connection with the band.

Then, Paul Buchanan sheepishly told the crowd that they'd played everything they knew, but would be happy to repeat a song or two if we'd like. The joint erupted in applause and screams, and The Blue Nile wowed us again.

Fabulous, unforgettable night.

-- Howard

How do doobs get fooled into relating experiences when that was NEVER asked for?
If we have Peabody’s wayback machine I’m going with any of Beethoven priemeres in Vienna or Rach playing any if his own Concertos.

I'm going for the night Liszt and Thalberg had a piano playing contest.