13 June 1838
Commandant Andries Hendrik Potgieter, one of the leaders of the Voortrekkers, obtained the territory between the Vet and Vaal Rivers from the Bataung captain, Makwana, in exchange for forty-nine head of cattle and the promise of protecting the Bataung against Mzilikazi, chief of the Matabele. The Voortrekker town Winburg (originally Wenburg) was subsequently founded on a tributary of the Vet River. There is some controversy about the actual date of this event, some sources state the date as somewhere in 1836.
References

Muller, C.F.J. (ed)(1981). Five Hundred years: a history of South Africa; 3rd rev. ed., Pretoria: Academica, p. 165.| Potgieter, D.J. et al. (eds)(1970). Standard Encyclopaedia of Southern Africa, Cape Town: NASOU, v. 9 p. 66.| Potgieter, D.J. et al. (eds)(1970). Standard Encyclopaedia of Southern Africa, Cape Town: NASOU, v. 10 p. 608.