Petrov Day is a tradition celebrating Soviet military officer Stanislav Petrov, who played a key role in preventing a nuclear attack during a false alarm incident on September 26, 1983.
Further reading
Yudkowsky, Eliezer (2007) 9⁄26 is Petrov Day, LessWrong, September 26.
Related entries
human extinction | nuclear warfare | Stanislav Petrov | Vasili Arkhipov