In Greek superstition, Tuesday is considered unlucky when it falls on the 13th of the month.
The belief is known as triskaidekaphobia combined with the Greek association of Tuesday with Ares, the god of war. Greek tradition also links Tuesday with the fall of Constantinople on Tuesday, 29 May 1453, an event remembered as a national catastrophe. This historical memory helped give the weekday a darker reputation.
Greek-speaking cultures therefore differ from much of the English-speaking world, where Friday the 13th is the better-known unlucky combination. In Spain and many Spanish-speaking countries, Tuesday the 13th is also traditionally feared, although the historical explanations differ by culture.
The superstition does not mean that every 13th day is unlucky in Greece. The specific combination of the weekday and the number is the important detail, and the belief is cultural rather than a prediction supported by evidence.