Today is an unlucky day in Spain: Tuesday the thirteenth. Sort of like Friday the thirteenth. Why Tuesday? Because in Spanish -- being a Romance language -- it's Martes, the day of Mars, the god of war, and war is bad for you.
In general, since Classic days, Tuesdays have been considered bad luck in the former Greco-Roman empire, and perhaps Tuesday the thirteenth shows some sort of cross-cultural fertilization (or pollution, depending on who you talk to).