One of the most important features of 'Viking Age' warfare is its seasonal nature. In the overwhelmingly agrarian economy of early medieval Northern Europe, warfare was carried out pretty much exclusively between approximately April and August. Before then, men were needed for ploughing and sowing, and it was imperative for nearly all concerned that fighting men be home in time to carry out the harvest in late summer. For the Vikings coming from Scandinavia, there was the added impetus of heavy storms in the North Sea and Irish Sea in winter which would have made travel excedingly risky. Indeed, the Anglo-Saxon Chronicle records on at least a couple of occasions that relatively severe Viking invasion forces are scuppered when they are hit by heavy storms in the English Channel or the North Sea.

What this means is that in winter, Vikings would simply stay at home while Viking raiding parties who had left it too late would often 'overwinter' rather than attempt a return voyage or to keep fighting. They would erect a camp and stay there, often for several months. In many cases this lead to them trading with the local population (they would usually try to overwinter away from areas they had raided) and even putting down roots. Indeed the phenomenon of Norse settlement in the Danelaw is anticipated by an increased tendency for Viking forces to 'overwinter' in the UK. Usually the camps are rudimentally fortified with a basic pallisade or are re-occupied hillforts. At Repton, where the Vikings overwinter after their victory in 873, a D-shaped pallisade is erected around the local church which then functions as a longhall-cum-gatehouse. Sometimes, smaller parties simply lodge in a town or city alongside the residents.

This seasonal system is rigidly adhered to, and one of the chief reasons why the Viking attack on Wessex in 878 takes the usually well-prepared Alfred completely by surprise is because it is launched at Midwinter, where all standing garrison forces would have been sent home.