It has always been my understanding that the tradition of Boxing day originates from when many working people were "in service" or apprenticed or otherwise worked away from home and had very little time off. If they were household servants, they couldn't have Christmas day off as they were needed to look after the family and guests. Boxing Day was the one day over the Christmas period that they were allowed to go home to visit their families, taking with them "Christmas Boxes" of gifts, generally donated by the wealthier families who employed them. Hence the tradition of cold meat pie and left over turkey for Boxing day - it was the cook's day off!