11 May 1909
Following the National Convention held by four British colonies, namely, Transvaal, Orange, Natal and Cape, the draft South Africa Act was signed in Bloemfontein after fierce debate. The different laws concerning non-White political rights in the colonies made the franchise question very contentious. Natal, who feared to lose its English identity in a predominantly Afrikaner state, favoured a federation. The Convention, on which the government and opposition parties of the colonies were represented, settled on the terms of a union rather than a federal state. White South Africans welcomed the bill, but virtually all politically conscious Blacks objected to the provisions concerning them. W.P. Schreiner led a deputation of Black and Coloured leaders to London to appeal to the British government to change the non-White franchise provisions, but the mission did not succeed in its purpose. The draft South Africa Act was taken to London by a SA delegation. After the bill had been passed by the British Government, it was signed by King Edward VII on 20 September 1909 and it came into force on 31 May 1910. The constitution in its final form had some federal traits in the South African Act, notably the provincial system.
References

https://africanhistory.about.com/library/thisday/bl-ThisDay0511.htm Wallis, F. (2000). Nuusdagboek: feite en fratse oor 1000 jaar, Kaapstad: Human & Rousseau; Muller, C.F.J. (ed)(1981). Five Hundred years: a history of South Africa; 3rd rev. ed., Pretoria: Academica, p. 379.