The restaurant where I work has been collecting order forms for king cakes for the past few weeks. The other night, a woman who had recently moved to the States asked me about the cake and its importance to American culture. Unfortunately, all I could tell her at the time was that it is served during Mardi Gras and is very popular in New Orleans. But the cake's history actually starts way back in Europe.
In the book "Mardi Gras, gumbo, and zydeco: readings in Louisiana culture," Marcia Gaudet writes an essay about today's king cake and the European Epiphany cake from which it evolved. The Feast of Epiphany is celebrated in many Western branches of the Christian faith on January 6, the proverbial "twelfth day of Christmas." It commemorates the day when the three wise men---also called magi, or kings---arrived in Bethlehem bearing gifts for the baby Jesus. Epiphany is also the start of the traditional Mardi Gras season.
King cake is traditionally a yeast-based sweet bread baked in the shape of a crown, covered with white icing and gold, purple and green sprinkles---the official colors of the carnival. Although Mardi Gras itself can be traced back to the medieval ages, the colors weren't chosen until 1872. Gold represents power; purple is for justice, and green represents faith.
Another key element is the inclusion of a trinket inside the cake. The trinket is often a tiny baby figurine that represents the baby Jesus, but it can also be a bean, an almond, a horseshoe or many other things. Whoever gets the token in their piece is considered the king—or queen—and becomes responsible for the next king cake. Of course, this custom varies from place to place and family to family.
Although it's not clear when or why the cake tradition migrated from Epiphany to later in the Mardi Gras festival, Gaudet theorizes that it has to do with other Epiphany-related customs, such as gift-giving, being observed in conjunction with the Christmas holiday.
I was surprised to learn that unlike other tradition-centric holiday foods, the king cake is usually bought rather than made at home. (If you're feeling ambitious, though, here's a recipe.) Even Gaudet's grandmother in New Orleans did not make her own---in an 1899 diary entry, she wrote that she and her aunt picked up a king cake at the store for King's Day.
The cake has made the leap from New Orleans to other cities in the United States as the Mardi Gras celebration becomes more widespread. But I think Gaudet has the cake's popularity figured out: " also provides both Cajuns and 'newcomers' a means of participating in a food custom that is certainly easier to adapt to than eating boudin and crawfish."
I can't speak for boudin, which is a word used to describe various sausages used in Creole and Cajun cuisine, but I had a rather unfortunate experience with a crawfish during last year's Mardi Gras and won't be eating that again. A cake covered in frosting and sprinkles, however? No problem at all.