Every day Vikings most likely never used horned helmets in combat. No horned helmet was dug up to date (although one would have to wonder whether a cow horn would have survived a millennium), and fighting battles with horns attached to a helmet would have been, well, impractical. Why provide your enemy with the equivalent of two handles on top of your helmet? So they can conveniently grab and hold your head, while they knee you in the face, slit your throat, or worse?
Horned helmets, however, were used by Vikings for ceremonial purposes and rituals. A tapestry found in the Oseberg ship burial, and dated around the 9th century (at the height of the Viking age), shows a man wearing a horned helmet. More significant is the Vendal period Öland bronze plate (late 6th to early 9th century) shown above, which depicts a Úlfhéðinn wearing a wolf pelt, together with the god Óðinn, who also happens to be wearing a... horned helmet! More recently, a 5cm figurine of Óðinn wearing horns again was also found in a field near Mesinge in Hindsholm, on the island of Fyn in Denmark (soon to be on permanent display at Nationalmuseet in Copenhagen). There is also, of course, the Viksø horned helmets from the Bronze Age in Denmark. Last but not least, oath rituals with Skjǫldrinn have been involving horned helmet for more than 1,200 years.
Öland bronze plate