Fifth 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 fifth month of the year, corresponding approximately to the Roman and modern month of May, was called Thrimilchi. Bede, writing in 725, tells us:
Thrimilchi was so called because in that month the cattle were milked three times a day; such, at one time, was the fertility of Britain or Germany.
--Bede, On the Reckoning of Time, Chapter 15. Translated by Faith Wallis.
This might be literally true. As far as I know, lactation is a continuous process so presumably a cow could be milked three times a day instead of the usual two if that happened to be convenient. It might also be a variant on the "Lost Golden Age of peace and Plenty Far Away and Long Ago" which humans have a tendency to look back to with longing, regardless of whether it really existed.
However, I would guess it probably reflects the sudden availability of fresh pasture and fresh dairy products after the long months of winter. In lowland areas of Britain the grass usually starts to grow by April - which end of the month depends on latitude, local climate and weather - and is getting quite lush by May. In areas with upland grazing, May marks the time of year when livestock can be moved up to hill pastures, freeing up the low valley fields to be cropped for hay (or, these days, silage). In the Lake District, Pennine hills and North York Moors the snow has gone even from the summits by May in most years and the vegetation is starting to grow strongly enough to withstand the attention of hungry sheep and cattle. Even in the Scottish Highlands, further north and higher altitude, the snow has melted and the grass started to grow again by May in all but the highest corries.
After the long dark months of winter, when animals and people had been living on probably limited and almost certainly rather dull supplies of stored hay and dried or salted produce, the sudden appearance of milk, butter, cream and cheese must have been a most welcome addition to the diet. It would not be surprising if it was commemorated in the name of the month.
Bede: The Reckoning of Time. Translated by Faith Wallis. Liverpool University Press, 1999, ISBN 0-85323-693-3.