I was thinking about 'The Carnival of Swing' 29-5-1938 on Randall Island (NY). 25 bands in the line up. Duke Ellington, Count Basie, Stuff Smith (with Ben Webster), Artie Shaw, Ella Fitzgerald, and Gene Krupa as the closing act. But also Kay Kaiser and Vincent Lopez. Great bands for shure but jazz?. I could not find a complete line up.
Missing from the line up: Benny Goodman he played at the Steel peer (Atlantic City) that day.
Count Basie & His Orch. 1938 Randall's Island NYC - YouTube
Carnival of Swing: Uncovering an Historic Jazz Concert at Randall's Island Stadium, 1938 | The New York Public Library (nypl.org)
The poster 1938 Carnival of Swing Poster | Growling Hamster is a recreation of how it might have looked.