According to the Bain New agency, which took these photos of a duel with wax bullets, the date was October 1909 Oct. 28. But an article in the New York Times tells of a duel with wax bullets on February 26 1909.
For a brief time wax duelling was popular. It was featured as an associate (non-medal) event during the 1908 Summer Olympics in London.
Via: George Grantham Bain Collection