Inhabited from the dawn of time by the San, also known as the "Bushmen", invaded by the Bantu, colonized by the Germans who called it "South West Africa" and taken over by South Africa after WWI, Namibia is in many ways quite similar to South Africa. Since it was ruled under the apartheid system, Namibia also has many of the problems resulting from that system.
It is important to be aware that race is a common part of Namibian discourse. That is to say, Namibians will refer to the race of others more frequently than travellers from places where race is typically not an issue, would expect. Because of apartheid, race is an issue in many spheres of life, so it comes up a lot. In spite of this, the various races do get along well in Namibia, and it is fairly uncommon to find racial tensions flaring. Apartheid was never implemented as strictly in Namibia as in South Africa, so racial tensions are generally lower.
Namibia is similar to South Africa, and if you're used to travelling in one country, travelling in the other country is quite easy. There are some subtle differences. For example, in South Africa a non-white person may choose to speak English rather than Afrikaans as a political choice whereas among Namibia's mixed-race population who call themselves 'colored' in Namibia and South Africa Afrikaans is a proud part of their culture, and many people still speak German. Overlooking these differences isn't going to cause offense, but they're handy to know.
The public holidays in Namibia are: