|"November" by Jean Colombe. Tres Riches Heures of Jean, Duke of Berry, c. 1413|
"Mast" as used here, is the fruit of forest trees, such as acorns (from oaks), beechnuts, and chestnuts; a mast year is one in which the trees produce and drop an abundant crop, which litters the forest floor almost ankle deep. It may seem odd that the farmer would so worry about the likelihood of a crop of acorns or beechnuts that he would watch the weather for it, but for centuries, mast has been an important food source for animals which supply meat for the table - not only domestic hogs and cattle, but also game animals such as deer, wild boar, and grouse. A year of little or no mast would mean lean hogs and scarce game.
Because of the plentiful supply in a mast year - more than could be eaten - some of the nuts and acorns would have a chance to germinate and become trees, equally important at a time when the fuel supply was mainly wood.
In the picture above, a peasant is engaged in one of the activities of late autumn - knocking down such nuts as remain in the trees, while his hogs gorge themselves on the fallen fruit. The hogs would have been turned out of their pens in late summer to forage in the forest throughout the autumn months; once fattened, they would be rounded up and either driven to market or slaughtered for future consumption.
Pioneer farmers of the New World put up fences to protect growing crops from animals - wild or domestic - and let their hogs roam free, sent out with the injunction "Root, hog, or die" (the hogs rooted). As had their forebears, the farmers rounded up their fattened hogs in the late fall, and then it would be time to turn some of them into smoked ham and other good things.
For those who depended on acorns as a staple of their diet, like the Indians of North America, a mast year would be hailed in much the same light as a year of bumper crops to the farmer. Not only would they have sufficient meal for the coming year, but game would be plentiful as well.
How it was determined that rain today forewarned the farmer that food for his hogs would have to come out of his own store rather than the forest is anybody's guess. I'm sure that the years in which the prognostication was proved false were filled with great rejoicing.
Mast as food for hogs is making a (slow) comeback; this article contends that feeding hogs on mast is not only more humane, but produces healthier meat.
If you are interested in how acorns are turned into meal (with a couple of recipes), read "Acorns: from Mush to Candy".