December calendar images typically feature a pig slaughter, a common labor for December. In the case of the fragmentary prayer book from The Walters museum, Walters W. 425, a pig being butchered was the image for November. December calendar images, when they don’t feature a hog being butchered, often feature a boar hunt. Sometimes December calendar images feature a winter scene, or, sometimes, bread baking. The December calendar image from Walters W. 425 f. 12r has a medallion in the margin featuring the astrological sign of Capricorn, the horned ram, and below it, a winter scene featuring a snowball fight, with a rickety windmill (yes, this is Flemish) in the background.
Other than their clothing, the children look very much like similar children engaged in a snowball fight today. I love the realistic detail of the snowball splats on their clothing. Notice that the kid in green is clearly targetting the person with the horse (donkey?), while the kid in red has an off-scene target. I also very much like the “in the moment” aspect of the image. The figure in the lower right is bent over, scooping snow, while both his ppers are in mid-throw. Notice that no one is wearing gloves or mittens.