holiday sweets are central to the observance of secular and commemorative occasions in societies throughout the world. Although all holidays are at heart commemorative, recalling some significant religious or historical event, they tend to become self-commemorative as well, accruing new traditions that are connected only indirectly or not at all to the essential nature of the original event. This process encourages the development of increasingly local traditions. Another, related process is that of grafting, by which a holiday belonging to a newly adopted religion incorporates elements of older traditions. The spread of Christianity famously involved such cultural absorption: the Christmas tree resulted from the association of Christmas with northern European pagan practices. See christmas.