week ThirtY: King cake

King cake is a traditional portuguese cake eaten during Christmas time. It's made with a soft dough and filled with nuts, raisins and candied fruit to decorate. Recently, I learned that New Orleans has a King Cake as well. Theirs is eaten until Mardi-Gras and the same soft dough is used. Both cakes share another interesting fact - they both have hidden a trinket and tradition dictates that whoever gets the prize as to pay for next year's King cake!
Where did the tradition start?