The Day of the Death as such has its roots in the Aztec culture and the Spanish Invaders culture. It´s estimated that it started sometime in the past 2,000 to 3,000. More recently, however, the Aztecs incorporated it as a celebration of their godess Mictecacíhuatl, "The Death Lady". The Spanish Conquistadores could have care less, but when the missionaries arrived, in an effort to integrate the Native cultures (and thus make their evangelization easier) had to find ways to adapt the local traditions to more palatable Spanish (Catholic) ways. Specially the ones with a deep religious significance. That´s how the celebration came to be. It´s a watered down version of what the Aztecs did, mixed with as stated somewhere up-thread, the "Day of All Saints". November the 2th is the "Day of the Death" and November the 1st. is the "Day of All Saints."
The celebration more or less varies throughout Mexico but the central theme is the same, I suppose. You will find the more intense, colorful celebrations in Central Mexico with thousands of people attending and the South and a more somber, diluted version in the North. A couple of nice towns to be in for this are Pátzcuaro, in the State of Michoacán and Mextli, in the State of Mexico.
People will essentially built altars in their home honoring their death, with food, candy, local sweets, water, as a symbol of life, and Cempasúchitl flowers (a species of Marigolds) This flower is particularly important as it is believed it guides the spirits back home, which is the main point of the celebration. The actual day, people flock to cemteries where they will eat, drink, talk about and even dance on their deceased relative´s gravesite. The idea being that the spirit is there and is hearing everything and enjoying the party with the living. Favorite food, music, drink and clothing of the deceased is brought along and laid on top of the grave.
There are also foods that are also prepared for the occassion only, such as the "Bread of the Death" which is a sort of very flavorful, sweet, sugar sprinkled bread. Sugar eskeletons or skulls are prepared and eaten, mostly by children too. These are edible variations of the "Catrinas", which are highly decorated paper or cardboard skeletons with huge smiles and many, many variations of local dresses. The idea is to de-dramatize death and celebrate it in a way, while also celebrating life. Kind of complicated but that´s the idea.
The holiday is celebrated in other Latin American countries, such as Ecuador, Bolivia, Peru and Brasil, but the most sincretic and interesting is the Mexican one. And this is said by the Anthropologists not me.
You may wish to do a Google search for images, which is not difficult and YouTube videos. You already have the key words for your search.
I have never heard of Venom Diagrams. Let me know if you are interested in some more info.
[Edited 2012-10-31 10:36:57]