It's that day

Triskaidekaphobia (from Greek tris meaning "3", kai meaning "and", deka meaning "10" and phobos meaning "fear" or "morbid fear") is fear of the number 13 and avoidance to use it; it is a superstition and related to the specific fear of the 13th person at the Last Supper being Judas, who betrayed Jesus.  The antichrist shows up in chapter 13 of the Book of Revelation.

It is also a reason for the fear of Friday the 13th, called paraskevidekatriaphobia (from Παρασκευή Paraskevi, Greek for Friday) or friggatriskaidekaphobia (after Frigg, the Norse goddess after whom Friday is named in English).

As you may know, not all cultures have the Friday the Thirteenth problem. The Greeks, interestingly enough since the fancy word for the fear has Greek roots, think Tuesday is the unlucky day and that the number 13 is lucky. Tuesday the Thirteenth, however, is considered unlucky. Very confusing.

By the way, the fancy terms did not exist in ancient Greece but were created in the twentieth century. 

Spanish-speaking countries also often think of Tuesday the Thirteenth the way Americans think of Friday the Thirteenth. This may be because the Spanish word for Tuesday, martes, derives from Mars, the Roman god of war. And war is always unlucky. Next week, by the way, will be Mardi Gras [Fat Tuesday], since the French word for Tuesday (Mardi) has the same Latin root as the Spanish. 

There is also a tradition that Adam and Eve ate the apple on Tuesday. Don't know why.

If Friday the Thirteenth does bother you, however, you will not be pleased to know that 2015 contains three such dates. The next one is coming fast: March 13. Then you can breathe easily until November.