December 21 marks the winter solstice. Shortest day of the year. First day of winter. Keystone of astronomy and ancient religion.
I don't know the history or symbology of this and I can't seem to find anything online (Wikpedia and Google failed me). But it was a pretty good sales pitch from a shop owner who got me to buy to bookmarks, posters and cards with 'seasonal' symbols on them. I was dooped!