16 December: Day of the Vow
Boers traditionally celebrate the 16th of December as the Day of the Vow, remembering the day in 1838 when a group of Voortrekkers defeated a Zulu army at the Battle of Blood River

 BLOOD RIVER Vow (1838):

My brethren and fellow countrymen, at this moment we stand before the holy God of heaven and earth, to make a promise, if He will be with us and protect us and deliver the enemy into our hands so that we may triumph over him, that we shall observe the day and date as an anniversary in each year and a day of thanksgiving like the Sabbath, in His honour; and that we shall enjoin our children that they must take part in this, for a remembrance even for our posterity; and if anyone sees a difficulty in this, let him return from this place. For the honour of His name shall be joyfully exalted, and to Him the fame and the honour of the victory must be given.

When Apartheid ended and the African National Congress under Nelson Mandela came into power in South Africa in 1994, the national holidays were changed to days that would be meaningful to all South Africans.

http://africanhistory.about.com/library/bl/blsaholidays.htm