Query: What date did Estonia join the United Nations?
Context: Estonia was a member of the League of Nations from 22 September 1921, has been a member of the United Nations since 17 September 1991, and of NATO since 29 March 2004, as well as the European Union since 1 May 2004. Estonia is also a member of the Organization for Security and Cooperation in Europe (OSCE), Organisation for Economic Co-operation and Development (OECD), Council of the Baltic Sea States (CBSS) and the Nordic Investment Bank (NIB).