Is not exactly a festival, but is similar as it involves all the society towards a celebration, in this case is to venerate and welcome those persons who already passed away to another dimension and only comes back during this days.
The first of November is the celebration of the dead children and the second day is for all the grownups. Tradition says that the Monarch Butterfly is the soul of those persons and normally arrives to Mexico on the first days of November, the color of the wings: orange and black is the theme for the festivities as also the traditional flower of the day is also orange, an interesting coincidence, same as pumpkins.
In contrast with the celebration of Halloween this date is not to scare people or ask for trick or treat, unfortunately a lot of the tradition is being lost lately due to the globalization. The relation of the Day of the Death and the living people is one of rendezvous, the dead come on this day to visit and spend some time with their families.
The "Ofrenda" (offering) is setup a the tomb or at the house in front of a picture of the loved one. The offering involves all the food and drinks that the deceased one loved, sometimes there are even music bands or a mariachi playing for several hours. Is a time of rejoicement, and remembrance.