Today’s calendar reads Tuesday the 13th, a day considered by many to be the unluckiest of the year.

Tuesday is traditionally regarded as an occult day—one associated with misfortune and danger by superstitious people. When Tuesday coincides with the number 13, it is believed to be especially unlucky. In contrast, in other cultures the unlucky day is Friday the 13th.

In Hispanic cultures, the day is expressed in the proverb “En martes, ni te cases ni te embarques” (“On Tuesday, neither marry nor embark on a journey”).

A similar belief exists in Western traditions, where superstitious people avoid starting new work, traveling, or getting engaged on a Tuesday.

According to Greek tradition, Tuesday is considered a day of bad luck because Constantinople fell to the Ottoman Turks on a Tuesday—May 29, 1453. As noted by the “father” of Greek folklore, Nikolaos Politis (1852–1921), this belief predates later interpretations; contemporaries of the Fall already attributed the catastrophe to the ill influence of the day. Evidence of this superstition can be traced as far back as 1164.

Politis explained that the origins of this superstition lie in astrological beliefs. Tuesday is governed by the planet Mars, which symbolizes conflict and destruction. At a certain time of day—the so-called “evil hour”—Mars is believed to dominate alongside Saturn, making that period especially dangerous. Because this hour cannot be precisely identified, the entire day of Tuesday came to be regarded as unlucky.

The number 13 is considered supremely unlucky because it disrupts the harmony of 12—a number traditionally associated with completeness (the 12 Olympian gods, the 12 labors of Hercules, the 12 tribes of Israel, the 12 disciples of Christ, the 12 Imams, and others). By adding one to this complete cycle, 13 represents the unknown, which has long inspired fear and uncertainty. As a result, it became associated with misfortune.