This type of pan de muertos is made for the celebration of the Days of the Dead, November 1 and 2, when families honor their dead by setting up a colorful altar in the house and preparing special foods to put on it and take to the graveside. While this custom has its roots in pre-Columbian central Mexico, it has become popular even in the north (though a friend from Sonora says that her mother considered it very bizarre).
This semisweet yeast bread enriched with egg yolks and other similar breads are circular in form, transversed with "bones," and topped with a "skull." The same dough is used for the rosca de reyes, made for the celebrations for Twelfth Night, January 6. Don't try to hurry the dough along; the flavor will develop better with slow rising. I certainly advise an electric mixer with dough hook for this type of dough, which is rather messy to handle.