13 December 1880
Three years prior to this date, the British had annexed the Transvaal under Shepstone. Following this was a period of passive resistance and repeated attempts by Boer leaders to have the annexation of the Transvaal revoked. These attempts did not work. So on 8 December 1880 between 8 000 and 10 000 Boers gathered at Paardekraal, near Krugersdorp. A triumvirate of leaders were elected (Paul Kruger, Piet Joubert and M. W. Pretorius) and on 13 December 1880 the leaders resolved to restore the Transvaal Republic and three days later raised their Vierkleur flag at Heidelberg, thus rejecting British authority. The events of the 13 December 1880 thus in effect started the war and ended passive resistance. The war became known as the Anglo-Boer War 1 or the Transvaal War of Independence.
References

Wallis, F. (2000). Nuusdagboek: feite en fratse oor 1000 jaar, Kaapstad: Human & Rousseau.|Swart, M.J., et al. (eds) (1980). Afrikaanse Kultuuralmanak,  Aucklandpark: Federasie van Afrikaanse Kultuurvereniginge.|Potgieter, D.J. et al. (eds) (1970). Standard Encyclopaedia of Southern Africa, Cape Town: NASOU, v. 1, p. 381.