Besides Clara Zetkin, Käte Duncker helped propose Women's Day at the 1910 conference. Duncker was a German socialist and women's-rights activist who worked within the international socialist women's movement.
The proposal was made at the Second International Socialist Women's Conference in Copenhagen on August 26–27, 1910. Zetkin and fellow German delegates, including Duncker, called for an annual day focused on women's rights; the conference approved the idea without fixing a date.
Duncker is sometimes confused with other prominent socialist women such as Rosa Luxemburg or Alexandra Kollontai. Both were important figures in the broader movement, but the German delegate specifically named in this account is Käte Duncker. The first International Women's Day observances followed in 1911.