From: http://www.coolcleveland.com/blog/2011/10/dia-de-los-muertos-2011/

Ever since pre-Colombian times in Mexico, it is believed that our ancestors come to visit us once a year during "el Día de los Muertos." Not only is this a sacred time – it is also a time of celebration and joy – a time to honor the dead by preparing special offerings of food and drink, flowers, candles and incense.

The "Day of the Dead," one of the most respected holidays in many Latin American countries, is held in special regard in Mexico where the dead are remembered and honored. It is a time of happiness and celebration. The custom of honoring the dead is pre-Columbian in origin and originated as a harvest celebration in the month of September or October.

The first crops of the fall harvest – such as the first beans, corn and squash -- were not eaten. Instead, they were prepared as offerings to Tonantzin, our mother earth, to give thanks for the gifts that she had given us. It also was a time to share offerings of food and drink with our ancestors who came to visit from the underworld. The arrival of Catholicism after the conquest caused several changes to the indigenous traditions that affected the date of the celebration. Since that time, the Day of the Dead has been celebrated on the first two days of November.