UN Day marks the anniversary of the entry into force in 1945 of the UN Charter. With the ratification of this founding document by the majority of its signatories, including the five permanent members of the Security Council, the United Nations officially came into being.
24 October has been celebrated as United Nations Day since 1948. In 1971, the United Nations General Assembly recommended that the day be observed by Member States as a public holiday.
U.N. Day has traditionally been marked throughout the world with meetings, discussions and exhibits about the achievements and goals of the organization. In 1971, the General Assembly recommended that member states observe it as a public holiday.
Several international schools throughout the world also celebrate the diversity of their student body on United Nations Day (although the event is not necessarily celebrated on 24 October). Celebrations often include a show of cultural performances in the evening and a food fair, where food is available from all over the world.
Sources: un.org; wikipedia.org