8 May 1925
When the Union of South Africa was formed in 1910, it consisted of the Transvaal, the Orange Free State, Natal and the Cape Province. Dutch and English were the two official languages. However, there were attempts to gradually replace Dutch with Afrikaans. On 23 April 1914 C.J. Langenhoven, member of the Cape Provincial Council proposed that Afrikaans should replace Dutch as the medium of instruction in all primary schools up to standard IV. His proposal was unanimously adopted by three provinces but rejected by Natal. In 1919 the Dutch Bible was translated into Afrikaans. On 8 May 1925, the Official Languages of the Union Act No 8 of 1925 was passed at a joint sitting of the House of Assembly and the Senate. By this Act Dutch was replaced by Afrikaans. Both Afrikaans and English enjoyed equal status and rights.
References

Giliomee, H., (2003), 'The Rise and Possible Demise of Afrikaans as a Public Language', from praesa, [online], Available at https://www.praesa.org.za/files/2012/07/Paper14.pdf [Accessed 27 May 2014]|Afrikaners, The Culture., 'Countries and Their Cultures', from every culture, [online], Available at https://www.everyculture.com/wc/Rwanda-to-Syria/Afrikaners.html [Accessed 27 May 2014]