The following bit of history is from The King Arthur Flour 200th Anniversary Cookbook. The book is an outstanding example of what a cookbook should be, and by far the finest one I've ever seen when it comes to any sort of bread. I think they're more experts on baking than history, but so far everything else they've written about bread has proven to be right. So why not 'history of bread'?
Scottish shortbread was originally made from oatmeal and was served on the Winter Solstice, the shortest day of the year. The edges of the round "cake" were notched, symbolizing the sun which was being entreated to return. Nowadays in Scotland, shortbread is mostly made with wheat flour but the edges are still marked with those symbolic notches. It is served on Hogmanay (New Year's Eve) and New Year's morning to "first-footers," those revelers who have stayed up all night to see the New Year and are the first to go from house to house, visiting and celebrating.