Tenth 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 tenth month of the year, corresponding approximately to the Roman and modern month of October, was called Winterfilleth.
Bede, writing in 725, tells us:
But originally, they divided the year as a whole into two seasons, summer and winter, assigning the six months in which the days are longer than the nights to summer, and the other six to winter. Hence they called the month in which the winter season began "Winterfilleth", a name made up from "winter" and "full moon", because winter began on the full moon of that month.
--Bede, On the Reckoning of Time, Chapter 15. Translated by Faith Wallis.
This would place the beginning of Winterfilleth at the first full moon after the autumn equinox. The equinox marks the point at which the night and day are of exactly equal length, so after the autumn equinox the nights are longer than the days. The autumn equinox falls on around 22 September in the modern calendar, so Winterfilleth would begin in late September or early October, depending on the phase of the moon relative to the solar year. (See the Trilithi article for an introduction to the difficulty of managing a calendar with lunar months and a solar year).
Bede's statement indicates clearly that the early English lunar months were reckoned from full moon to full moon, rather than at some other point of the lunar cycle (such as the new moon, or the first crescent, or whatever). It is possible to argue that for some reason the season of winter began at the full moon and the month began at some other point, but this seems unnecessarily complicated to me.
Bede: The Reckoning of Time. Translated by Faith Wallis. Liverpool University
Press, 1999, ISBN 0-85323-693-3.