December calendar pages in books of hours typically feature butchering pigs, baking, and sometimes, both at once in the form of roasting a slaughtered pig. The porcine emphasis in December is a reasonable one, given that the pigs fattened by eating mast in the form of nuts and acorns in November are now ready to be butchered and roasted.
The image below is a detail from the Très Riches Heures of Jean, duc de Berry (Chantilly, Musée Condé, MS 65) calendar image for December. It features a wild boar hunt. The building in the background is the Château de Vincennes, where the Duc de Berry was born in 1340, on November 30. The forest bordering the estate was famous for its game (and was reserved as a royal forest). The leaves are still on the trees, though they do suggest late autumn, on the cusp of winter.
The boar has been cornered, speared by the huntsman standing off to the side, and is being destroyed by boar hounds. The realism of the dogs is astonishing; there are both boar hounds, and smaller bloodhounds.
On the right side of the image another huntsman blows the mort, or death call, on his small horn. It doesn’t look terribly wintery, I admit, though you’ll notice the huntsmen are not dressed for summer. But December serves as a good time for a boar hunt (see for instance the boar hunt featured in Sir Gawain and the Green Knight) and for domestic swine butchering because it is cold, and because the wild boar, like his cousin the domestic hog, has been eating fattening on nuts acorns.