Not seen it before but an interesting find. I read somewhere that Arthur Ackermann had the idea of producing Christmas cards by using his existing trade cards and replacing the advertising with a Christmas message. He also sold the idea to an american company (Prang?) in the 1870s. The printed message at the bottom is a quite well known quote, and while it could be presaging the conflict in Zululand that is pretty sombre for a Christmas greeting if you knew what had happened, so I would go for 1878. It is certainly depicting a Zulu so who knows.