Celebrated each year to mark the day on which the United Nations came into existence on 24 October 1945, when the Charter was ratified by China, France, the Soviet Union, the United Kingdom, the United States and by a majority of other signatories. United Nations Day is celebrated on 24 October each year.

http://www.un.org/events/unday/
http://www.holidayorigins.com/html/united_nations_day.html