La galette des rois is a cake to celebrate the Epiphany, a Christian feast day for the coming and incarnation of Jesus Christ. Though traditionally on January 6, but because it is not a public holiday, the feast day occurs on the first Sunday of January (unless it falls on January 1st).
Since the 14th century, people in France eat the galette des Rois once a year. According to the tradition, the cake must be divided so that each guests gets a slice, plus an extra one called the part du Bon Dieu/Vierge/Pauvre (Good Lord/Virgin/Poor) which is reserved for any unexpected stranger.
The cake is typically bought in a boulangerie, and is made of pâte feuilletée (puff pastry), frangipane (filling made from or flavored like almonds) or brioche (sweet bun). A fève (charm) in a shape of a figurine is hidden in the cake.
When kids are present, one of them (generally the youngest), must go under the table and directs whoever is serving to whom each slice should be given. The lucky one who gets the figurine becomes the king or the queen of the day, and he/she is given a golden or silver couronne (crown).