Eleventh month of the Old English calendar
For an overview of the Old English calendar and links to the other months, see the summary page.
The eleventh month of the year, corresponding approximately to the Roman and modern month of November, was called Blotmonath, "blood month".
Bede, writing in 725, tells us:
Blodmonath is "month of immolations", for then the cattle which were to be slaughtered were consecrated to their gods. Good Jesu, thanks be to thee, who hast turned us away from these vanities and given us to offer to thee the sacrifice of praise.
--Bede, On the Reckoning of Time, Chapter 15. Translated by Faith Wallis.
As a good Christian, Bede clearly disapproved of animal sacrifices to heathen gods. There is another famous reference to cattle sacrifice in Pope Gregory's advice to Bishop Mellitus on how best to approach the conversion of the English to Christianity:
And because they have been used to slaughter many oxen in the sacrifices to devils, some solemnity must be exchanged for them on this account, as that on the day of the dedication, or the nativities of the holy martyrs, whose relics are there deposited, they may build themselves huts of the boughs of trees, about those churches which have been turned to that use from temples, and celebrate the solemnity with religious feasting, and no more offer beasts to the Devil, but kill cattle to the praise of God in their eating.
--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.
Bede: The Reckoning of Time. Translated by Faith Wallis. Liverpool University Press, 1999, ISBN 0-85323-693-3.
Bede, Ecclesiastical History of the English People, Translated by Leo Sherley-Price, Penguin, 1990, ISBN 0-14-044565-X.