Ninth 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 ninth month of the year, corresponding approximately to the Roman and modern month of September, was called Halegmonath, "holy month".
Bede, writing in 725, tells us:
Halegmonath means "month of sacred rites".
--Bede, On the Reckoning of Time, Chapter 15. Translated by Faith Wallis.
He doesn't elaborate, which is a shame. So we do not know why the month was called holy, nor what rites were performed or what deities honoured. I think a few inferences can be made, though (as ever) other interpretations are possible.
In most of temperate Europe, the main cereal crops are harvested during August and September and harvest is completed some time during September, depending on the weather and the crop (for example, barley is harvested earlier than wheat in regions where both are grown). Cereal crops, such as rye, oats, barley and the various types of wheat, were the staple food before potatoes were introduced from the New World. More than any other crop, the cereal harvest determined whether the ensuing winter would be a hungry one. The month in which the cereal harvest was safely gathered in and the future of the community secured for another year, could justifiably be considered a holy month.
What deity might have been honoured in this holy month? Tacitus says of the Angles in continental Germany in the first century AD:
There follow in order the Reudignians, and Aviones, and Angles, and Varinians, and Eudoses, and Suardones and Nuithones; all defended by rivers or forests. Nor in one of these nations does aught remarkable occur, only that they universally join in the worship of Herthum; that is to say, the Mother Earth. Her they believe to interpose in the affairs of man, and to visit countries. In an island of the ocean stands the wood Castum: in it is a chariot dedicated to the Goddess, covered over with a curtain, and permitted to be touched by none but the Priest. Whenever the Goddess enters this her holy vehicle, he perceives her; and with profound veneration attends the motion of the chariot, which is always drawn by yoked cows. Then it is that days of rejoicing always ensue, and in all places whatsoever which she descends to honour with a visit and her company, feasts and recreation abound.
The goddess' name is variously rendered as Nerthus, Ertha or Herthum depending on the translation. The original Latin is, "Nec quicquam notabile in singulis, nisi quod in commune Nerthum, id est Terram matrem", so I'll use the form Nerthus.
A Mother Earth goddess would seem to be a reasonably likely candidate for a deity to be honoured in a month that celebrated the grain harvest.
Kathleen Herbert quotes from an account written by a German visitor to southern England in September 1598:
By lucky chance we fell in with the country-folk celebrating their harvest-home. The last sheaf had been crowned with flowers and they had attached it to a magnificently robed image, which perhaps they meant to represent Ceres. They carried her hither and thither with much noise; men and women were sitting together on the waggon, men-servants and maid-servants shouting through the streets until they came to the barn.
--Quoted in Herbert (1994)
Ceres was the Roman goddess of agriculture, from whose name we get the modern English word "cereal".
So 1500 years after Tacitus described Nerthus riding in a ceremonial vehicle amid great rejoicing, we have an account of the English celebrating the corn harvest in September by carrying a female image in a waggon, also amid noisy rejoicing. It should be noted (and should go without saying) that the 1598 account doesn't prove an uninterrupted survival of ritual, much less religion, for 1500 years. For all the German visitors (and we) knew, the English villagers might have invented their celebration the year before based on a fragment of Roman myth that someone had seen or heard of and thought would make a good excuse for a party. Nevertheless, it may not be too far removed from the "sacred rites" of the early English "holy month".
Tacitus, Germania. Full-text translation available online.
Bede: The Reckoning of Time. Translated by Faith Wallis. Liverpool University Press, 1999, ISBN 0-85323-693-3.
Herbert, Kathleen. Looking for the Lost Gods of England. Anglo-Saxon Books, 1994. ISBN 1-898281-04-1.