25 May 1963
On 25 May, 1963, the first African organisation after independence, the Organisation of African Unity (OAU) was formed in Addis Ababa, Ethiopia. The organisation was founded by thirty-two countries, a further 21 states have joined gradually over the years, with South Africa becoming the 53rd member on 23 May 1994. The OAU started its operations on 13 September, 1963, when its Charter was adopted. The OAU was committed to helping bring about change and freedom to many African countries and restore dignity of African people. The 25th May, now celebrated as Africa Day, marks the beginning of a quest for the unity of the continent and for the political and economic emancipation of its people as well as co-operation among them.  It is at this historic meeting that the date of Africa Freedom Day was changed from April 15th to May 25th. In 2002 the OAU was dissolved and was replaced by the African Union (AU).
References

‘Organization of African Unity (OAU)’, from Encyclopedia.com [online], Available at www.encyclopedia.com [Accessed: 26 April 2012]|‘Organisation of African Unity’, from South African History Online, [online], Available at www.sahistory.org.za [Accessed: 26 April 2012]|‘First celebration of Africa Day’, from South African History Online, [online], available at www.sahistory.org.za [Accessed: 25 April 2012]