The Carpenters at Universal Studios in 1971. I was in junior high.
In what country?
The first US concert I attended was the Beach Boys, Lexington, KY, 1970, my freshman year of college.
Do Christmas concerts at church or school count?
I was envisioning concert hall/stadium as opposed to a school or non professional.
Why not? It was not specific enough so even a garage band will count as an answer... so a church organist too.