I was at Zep's very first concert in the USA. They opened for Spirit and Vanilla Fudge.
"I got a line on you" was just getting airplay, and Keep Me Hangin On" was huge on the radio.
My sister and I were blown away by Zep, they played most of the first album. We called the radio station the next day.
We wanted to know more about the band, the radio station said their album wasn't out yet....the next month, everyone I knew had it!
I still have the ticket stub. I was in maybe 7th grade.
Edited to show Denver Post Review pic