United Nations Day is observed on October 24th, it is not an Official US (Federal) holiday.
The UN officially came into existence on October 24th, 1945 upon ratification of the United Nations Charter by the five permanent members
of the Security Council (France, the Republic of China, the Soviet Union, the United Kingdom and the United States), and by a majority of
the other 46 signatories.
The United Nations was created after WW-II as a successor of the failed League of Nations (1919 - 1946), an organization which the
United States never joined. The United Nations was established in 1945 with the aims to maintain international peace and promote
cooperation in solving international economic, social and humanitarian problems.
Franklin D. Roosevelt first coined the term 'United Nations' as a term to describe the Allied countries. The term was first officially
used on January 1, 1942 when 26 governments signed the Atlantic Charter, pledging to continue the war effort. On 25 April 1945, the UN
Conference on International Organization began in San Francisco, attended by 50 governments and a number of non-governmental organizations
involved in drafting the Charter of the United Nations.