Jose Guadalupe Posada´s "Calacas" etchings have shown skeleton parties since the ends of the XIX century,
The other hard influence on this specific work was the 1929 silly symphony "The skeleton dance" created by "Mickey Mouse" designer Ub Iwerks. It´s a great animation piece from a time when Disney took chances on the animation format.
Other great Image I just found on a memorabilia blog that collects the marvelous works of Rankin and Bass. Gotta see this movie. From Mad Monster Party ( 1969) Directed by Jules Bass. (vía The Rankin-Bass Historian)
Background for the first shot of the project about the future I´m working on right now for UNICEF with my father Gustavo Wilches-Chaux . Pencil & Digital Coloring
Update: The proposal got scrapped. Still it'll work for something else.