While in Oaxaca, I kept asking friends and family of Marta to please explain the history of Day of the Dead, but no one knew! They all told me that this is a tradition that has been in their families and communities for centuries and centuries and that they continue the ceremony out of respect for the tradition and family members who have passed.
I found this neat website that explains the history, the symbolism and more of Dia de los Muertos. There were a number of other websites that were more informative, but all of them were in Spanish. This gives you a brief and overall description of the holiday-
Picture: Pan de muerto