à vis Britten is that on August 6, 1946, Leonard Bernstein conducted the American premiere of his opera Peter Grimes at the Tanglewood Festival in Lenox, Massachusetts. Boris Goldowsky, Tanglewood's music director, spoke presciently when he told Opera News that the work "has lasting merit, and it will join the standard repertory." Peter Grimes was the first opera Bernstein conducted professionally, and its famously atmospheric Sea Interludes were on the program of the last orchestral concert he ever conducted. That was in August of 1990—also at Tanglewood!
this blog post has links to special collections on Bernstein, Copland et al., as well as to the vast archives of the Koussevitzky Foundation, which commissioned Peter Grimes and many other great modern works.
To get a sense of the "musical utopia" that is Tanglewood in your own home, sample this PBS special on its 75th Anniversary Celebration, featuring the Boston Symphony, Boston Pops, Tanglewood Music Center Orchestra & Festival Chorus, John Williams, Keith Lockhart, Andris Nelsons, Peter Serkin, Yo-Yo Ma, and James Taylor.