The galette des rois is a cake traditionally shared at Epiphany.
It celebrates the arrival of the Three Wise Men in Bethlehem. Composed of a puff pastry, with a small charm, the fève, hidden inside, it is usually filled with frangipane - made from sweet almonds, butter, eggs and sugar.

They are everywhere right now.
I might wait a while to buy one though, just enjoying the lull after the Christmas food...