Blotmonath starts 29/10/11
The Month Of Sacrifice
-Bede, Ecclesiastical History, Book I Chapter 30 :-
When the grass stops growing in the late autumn or early winter in north-western Europe, the supply of food available for cattle falls dramatically. It is still possible to pasture a few animals outdoors, provided they are hardy enough to survive the winter weather, but the number will be limited because the vegetation that is already there has to last them until the new growth starts again next spring.
Keeping any larger number of cattle over winter requires the provision of winter fodder. This was traditionally hay, long grass cut in the lush days of summer and dried in the sun for winter storage. But hay is time-consuming to make, and in a wet summer it can be difficult (if not impossible) to dry it properly. The hay supply is also limited by the supply of grass available for cutting in the summer. All of this means the supply of food available for livestock during the winter would be a lot less than that available during the summer. Demand could be reduced to some extent if the cows went dry in the winter, as a cow needs less food when she is not producing milk. But even so, the number of cattle that could be kept in good health over winter would be limited.
Rather than let the surplus animals starve slowly to death, it would make sense to kill them while they were still in good condition, when some of the meat could be eaten fresh and the rest salted, smoked or dried to be eaten over winter. Hence an annual cattle slaughter in the late autumn would be required for sound agricultural reasons, and could provide a convenient opportunity to honour the gods (and have a big feast) at the same time. The god(s) might change, but the agricultural imperative stayed the same.