16 December 1963
Two African countries, Zanzibar (now part of Tanzania) and Kenya both became members of the United Nations on 16 December 1963. This year marks fifty years since the two African countries have been members. The organisation’s membership grew to 115 members that year. The United Nations currently boasts 193 members.  The organisation was founded in 1945 after World War II. Its main objective is to prevent war between countries and to provide a platform for dialogue.
References

United Nations, Growth in United Nations membership, 1945-present, from United Nations Online, [online] Available at www.un.org [Accessed: 25 October 2013]|Scribd, (2013), Zanzibar Becomes Member of the United Nations, [online] Available at www.scribd.com [Accessed: 25 October 2013]