I think one thing that surprised me when starting out researching various bands was that being able to confidently identify the first time they performed before an audience proved to be more difficult than I expected. Now, you would think that the first time you performed would have impressed itself on your mind if you are a musician and it does...just not the details. And that's why I do what I do.
So let's turn to a band that I've done several decade's worth of research on - Poco. This is a photo from their early days when they were known as Pogo...or R.F.D....or whatever they felt like that given night if you believed latter-day interviews (which I didn't). Check any given rock history and the Poco entry will tell you they made their debut as Pogo at the Troubadour in Los Angeles in November 1968. More detailed interviews with the band in the early 1970s revealed that their first performance was a hoot night - meaning they weren't billed, they went on as one of a number of unknown groups. This was clearly going to be difficult unless anyone with the group remembered the date...which they don't. Plowing through November 1968 issues of every Los Angeles newspaper also didn't provide any new information.
In delving through some early promotional material, I came across a press release from Epic Records that quoted several newpaper concert reviews...and one was a review that my research had not uncovered. Since I'd looked at every November issue, I expanded my search into October and sure enough, a review for a performance by R.F.D. on October 24th that included an early promotional photo of the band appeared. The writeup noted that the band had filled in that night for a band who had cancelled. The review also noted R.F.D. had debuted earlier at a hoot night under the name Pogo but they had since changed it. So October 24 was not their first show. But it did show that all the previous sources pointing to November as their debut were incorrect.
Additional research showed that Monday nights were traditionally hoot nights at the Troubadour so the best I can do is that October 7, 14, and 21 were the Mondays and on one of those nights, the band to ultimately become known as Poco made their debut.