The Roscon de Reyes is a traditional cake eaten on Spanish Christmas day which is on the 6th January. The history is not really related to Chrsitmas and goes back to Roman times where it was eaten for a slaves holiday in mid December. It was a reward for the hard work over the summer and given during the winter solstice an oportunity for the slaves to rest. It became a Christan ritual during the 4th century for 3 kings day.
Like the Christmas pudding where a the tradition is to hide a sixpence inside, now days the Roscon has a small plastic king hidden in one of the pieces and if you are the one that finds it, it means you have to pay for the desert. Hiding an item in comes from the Roman era where a fig bean was hidden inside. The slaves that found this would be given the day free from labour.
The bakeries will be full of these on the 5th of January