Some uniquely culturual events would have to be the Marulla Festival up North and the Olufuko Festival in Ombalantu. Independence day is celebrated on the 21 March, which is probably the most important day on the Namibian calendar. Independence festivities occur in every village, town and city and will often include singing and dancing as well as the obligatory speeches by members of parliament.
Maherero Day occurs in Okahandja on the weekend closest to the 26th of August. Thousands of Herero people gather in traditional dress for a memorial service to their chiefs, this is a spectacular and fun occasion. If you are going to be in Namibia around this time its worth visiting this festival, tours are often arranged from Windhoek.
Namibia also has several German style beer festivals the two most notable being Octoberfest which takes place in Windhoek during late October and WIKA (Windhoek carnival) which happens in April.
If you want to experience more of Namibia's culture it is probably worth visiting one of the community tourism endeavours.
"It is pretty clear now that what looked like it might have been some kind of counterculture is, in reality, just the plain old chaos of undifferentiated weirdness.", Jerry Garcia