The dry season between May and October is regarded as the best time to visit Namibia for game viewing. The skies are clear and the animals are concentrated around water sources.
The rains come in the summer, between November and April. The country turns green and birds flock to the saltpans. This is also a lovely time to be in Namibia as the desert rains are sporadic and unlikely to get in the way of enjoying your visit.
Of course, the dramatic sand dunes in the Namib desert can be visited at any time, and its worth noting that all of Namibia enjoys an average of 300 days of sunshine each year.