The souls of the dead are believed to come from THAT world to THIS only for 24 hours between November 1st, and 2nd. Everything is done to please the souls with the joys of life they used to love most of all. So, cemetery parties feature their favourite foods, drinks, and music. A dress-code includes traditional Mexican clothes and painted faces.
Besides food and family time, you can witness peculiar dances like La Danza de los Viejitos (the dance of the little elder men) during which young men and boys dress as old men, then walk around crouched and suddenly start an energetic dance.
It doesn't matter whether you don't belong to a celebrating family if you behave respectfully and follow common rules you will be welcome. It's especially interesting to observe if you are in a Mexican village, for local celebrations are more solemn than those held in cities.
Southern regions of Mexico are renowned for the brightest parties, particularly the states of Michoacan, Chiapas, and Oaxaca. The celebration starts already on October, 30, and lasts for three days.