In the second half of the 16th century the Little Ice Age had just begun to bite, and the winter of 1564-65 was the longest and most severe of any since the early 1430s. It was in February of that winter that Bruegel painted Hunters in the Snow, and in the few years immediately succeeding it he appeared to want to apply a layer of snow to everything he did. His idiosyncrasy started a new artistic tradition, and began the genre to which today's Christmas card producers almost universally conform.
There were, in fact, several very arty Bruegels, and to make things easier they all had nicknames. Jan, for example, who lived from 1568 to 1625 and painted landscapes, was known as "Velvet" Bruegel, both for his fondness for that luxurious material and for the smoothness of his brushwork. His brother Pieter had a penchant for mythology, and was known as "Hellish" Bruegel for the frequency of devils in his pictures. But the daddy of them all, in both a literal and figurative sense, was "Peasant" Bruegel. Pieter Bruegel the Elder recorded the merrymaking, the feasting and the daily lives of Flemish peasantry, and was one of the first, if not the first, to paint the landscape as a subject worthy of attention in its own right rather than as a schematic background to some other scene of religious or historical significance.
This was the Bruegel who underwent such a sudden change in style around the middle of the 1560s, taking to painting snow with great enthusiasm. His subsequent paintings were in marked contrast to his earlier work, most of which were set in exotic, almost summery conditions. Moreover, in later life Bruegel repainted many of his earlier pictures to make them look more "snowy": a case in point is The Massacre of the Innocents, which depicted a bloody biblical scene and to which Bruegel later added a layer of snow.
Some critics believe that this painting in its original form had so outraged the viewing public that Bruegel was obliged to add the snow to cover up the carnage and make it more acceptable. Meteorologists, however, prefer to believe that the weather at the time was the key to Bruegel's eccentricity. The relatively benign conditions of the first half of the millennium were followed by a significant drop in the average temperature of the northern hemisphere; winters became longer and more severe; and widespread snow was the rule and not just the exception. Our Christmas cards, therefore, are snowy for the simple reason that Peasant Bruegel painted what he saw.