- It was mid-November 1976, almost certainly between the 16th and the 21st.
- The ballet was Swan Lake. It had been rewritten to have a happy ending.
- It was in the Soviet Union.
Things I believe to be true, but could be mistaken about:
- It was at the Bolshoi. (It could also have been in Leningrad.)
- It featured their prima ballerina.
Anybody have any sorts of leads on where to find this information? I've googled a fair bit, but didn't strike paydirt.