The name "United Nations" coined by United States President Franklin D. Roosevelt, was first used in the Declaration by United Nations of 1 January 1942, during the Second World War, when representatives of 26 nations pledged their governments to continue fighting together against the Axis Powers.

In 1945, representatives of 50 countries met in San Francisco at the United Nations Conference on International Organization to draw up the United Nations Charter. Those delegates deliberated on the basis of proposals worked out by the representatives of China, the Soviet Union, the United Kingdom and the United States at Dambarton Oaks, USA, in August-October 1944. The Charter was signed on 26 June 1945 by the representatives of the 50 countries. Poland, which was not represented at the Conference, signed it later and became one of the original 51 Member States.

The United Nations officially came into existence on October 24, 1945, when the Charter had been ratified by China, France, the Soviet Union, the United Kingdom, the United States (the five Permanent Members of the UN) and a majority of other 46 signatories.

The anniversary of the entry into force of the United Nations Charter on 24 October 1945 has been celebrated as United Nations Day since 1948. It has traditionally been marked throughout the world by meetings, discussions and exhibitions on the activities and goals of the Organization. In 1972, the General Assembly decided to institute World Development Information Day, also on October 24, to coincide with the UN Day to draw the attention of world public opinion to development problems and the need to strengthen international cooperation to solve them.