The Day of the Dead is a national holiday in Mexico when people remember their friends and family who have died. Originating from an Aztec festival, now it is an excuse for a big 3 day piss-up.
The skull has become the main symbol for the festival and appears prominently in all types of Mexican art, which is awesome, because skulls kick arse.
Note, some of the images below are not necessarily associated with the Day of the Dead. They are some not too shabby masks from the pretty awesome Museo de Arte Popular, in downtown Mexico City.