During the summer of 1975 the important person in residence at Tanglewood was Mstislav Rostropovich, who came there with his wife Galena Vishnevskaya and their two daughters, Olga, who played the cello, and Elena, who played the piano. It was a very exciting summer, filled with master classes where Rostropovich would tell charming stories (I remember that his humor was rather unrestrained). Near the end of the summer Rostropovich was scheduled to perform Shostakovich's Second Cello Concerto. Many people knew the first concerto, but very few people knew the second, a piece that Shostakovich wrote for Rostropovich in 1966. Only nine years old, it was a piece of relatively new music. I am pretty sure that this performance was the first by the Boston Symphony, and it could have even been the first American performance of the piece. Anyway, it was a big deal.
The fact that Shostakovich's Fifth Symphony was on the second half of the program offset any chance of losing the audience to an unfamiliar work. Everyone loves Shostakovich's Fifth Symphony.
During the intermission there was a bit of a commotion on stage, and then Galena Vishnevskaya walked to the center of the stage and sang an unaccompanied lament. Everybody wondered what it was about. Eventually Bill Moyer, the Boston Symphony Orchestra's personnel manager, came on stage. He told the audience that Dmitri Shostakovich had just died.
Then Rostropovich conducted the performance of the Fifth Symphony. It was an awesome performance in every way. Many thousands of people were in a state of collective mourning. Wherever "there" is when a person dies, that's where we all were.
(I seem to remember that the program was an all-Shostakovich program, but a passage quoting Oliver Knussen's recollections of the day on page 188 of Peggy Daniel's Tanglewood: A Group Memoir suggests that there might have been some Tchaikovsky on the program.)