In Spanish-speaking countries, Tuesday the 13th is traditionally considered unlucky because of the number 13.
The belief is expressed in the Spanish saying martes trece, and it is comparable to the English-speaking superstition surrounding Friday the 13th. Tuesday is linked in part to Mars, the Roman god of war, whose name is associated with the day in Romance languages.
The number 13 has long carried an unlucky reputation in Western traditions. In Spanish and some other Mediterranean cultures, that reputation combines with the ominous associations of Tuesday rather than Friday.
The custom is cultural, not universal. People may avoid travel, weddings or important decisions on the date, while others treat it as ordinary. The exact strength of the belief differs among countries and individuals.