On this day in history, representatives of fifty nations meeting in San Francisco, California signed the 111-article Charter for a new international peacekeeping federation to replace the League of Nations. (Poland, which was not represented at the Conference, signed it later and became one of the original 51 Member States.)
There were 850 delegates, and their advisers and staff together with the conference secretariat brought the total to 3,500. In addition, there were more than 2,500 press, radio and newsreel representatives and observers from many societies and organizations. The heads of the delegations of the sponsoring countries took turns as chairman of the plenary meetings.
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 and by a majority of other signatories. United Nations Day is celebrated on October 24 each year.
You can read the text of the United Nations Charter here.