Before they converted to Christianity and adopted the Roman calendar, the early English (‘Anglo-Saxons’) reckoned time using a system of lunar months. Each cycle of the moon, probably from full moon to full moon, was a month. The year began at the winter solstice, the shortest day of the year. There were two seasons, summer, when the days were longer than the nights, and winter, when the nights were longer than the days (See my earlier post for a summary of the early English calendar.)
The eighth month of the year, corresponding approximately to the Roman and modern month of August, was called Weodmonath, meaning “weed month”.
Bede, writing in 725, tells us:
Weodmonath means “month of tares” for they are very plentiful then.
--Bede, On the Reckoning of Time, Chapter 15. Translated by Faith Wallis.
Anyone who has ever managed a garden knows that weeds are plentiful at more times than just August! Why pick on August as the weed month? It might be just a convenient name. The month has to be called something and ‘weed month’ might have been considered as good a name as any.
However, August is the time when the main cereal crops of temperate Europe – barley, rye, wheat, oats – are fully grown and ripening. The proportion of weeds in the cereal fields would be obvious by August. Perhaps it was a good indicator of (a) how difficult it was going to be separate the cereal from the weeds at harvest and threshing time and (b) the likely cereal yield; the higher the proportion of weeds in the cereal fields, the lower the yield of cereal. Maybe August was the weed month because it was then that you could judge how difficult the harvest was going to be?
Bede: The Reckoning of Time. Translated by Faith Wallis. Liverpool University Press, 1999, ISBN 0-85323-693-3.