An addition to node by bozon.

Dia de los Muertos is indeed a Mexican holiday. Whether it is the most important holiday is a point of debate.

Dia de los Muertos is largely a holiday celebrated in central Mexico and south. In northern Mexico, the holiday is largely ignored.

A possibly more important holiday is the celebration of the Virgen de Guadalupe, the patron saint of Mexico. The date for this is December 12th.

For an impressive list of Mexican holidays, see

http://www.eden.com/~tomzap/holiday.html