a clear sky to sing appropriate psalms and celebrate the Sabbath, taking a vow which became known as the "Day of The Vow or Covenant" that "if the Lord might give us victory, we hereby deem to found a house as a memorial of his Great Name at a place where it shall please Him", and that they also implore the help and assistance of God in accomplishing this Vow and that they write down this Day of Victory in a book and disclose this event to our very last posterities in order that this will forever be celebrated in the honour of God." On 16 December 1838, while laagered near the Umslatos River or Hippo Pool, they were attacked by more than 10,000 Zulus. As Bantjes wrote in his journal: The Boers fought off the Zulu. After three hours, the Boers had killed thousands of Zulus and had fewer than a dozen of their men wounded. The Zulus withdrew in defeat, many crossing the river which had turned red with blood and thereafter known as the Battle of Blood River. Modern-day Boers still celebrate the Day of the Vow every year on 16 December. British at Port Natal Returning south, Pretorius and his fighters found that the British had annexed Port Natal (now Durban) on 4 December with a detachment of the 72nd Highlanders from Cape Colony. While the governor of the Cape, Major-General Sir George Napier, had invited the British emigrants from Natal to return to the colony, and stated his intention to take military possession of the port. In sanctioning the occupation of the port the British government of the day had no intention of making Natal a British colony, but wished to prevent the Boers establishing an independent republic upon the coast with a harbour through which access to the interior could be gained. After remaining at the port just over a year the Highlanders were withdrawn, on 24 December 1839. Overthrow of Dingane After the battle, Pretorius took advantage of dissension in the Zulu kingdom to ally himself with Mpande, brother of the Zulu king Dingane. Dingane's attempt to extend his kingdom north to compensate for losses to the Boers had failed. He was defeated by the Swazi people in 1839, leading to discontent with his rule. In exchange for cattle and territory Pretorius agreed to support Mpande's bid to overthrow Dingane. A Boer force