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Both the number thirteen and Friday have been considered unlucky:

* In numerology, the number twelve is considered the number of completeness, as reflected in the twelve months of the year, twelve recognized signs of the zodiac, the twelve tribes of Israel, the twelve Apostles of Jesus, etc., whereas the number thirteen was considered irregular, transgressing this completeness.[2] There is also a superstition, thought by some to derive from the Last Supper, that having thirteen people seated at a table will result in the death of one of the diners.
* Friday, as the day on which Jesus Christ was crucified, has been viewed both positively and negatively among Christians. The actual day of Crucifixion was the 14th day of Nisan in the Hebrew Lunar calendar which does not correspond to "Friday" in the solar calendar of Rome. The 15th day of Nissan (beginning at Sundown) is celebration of Passover.

Similar superstitions exist in some other traditions. In Greece, Romania and Spanish-speaking countries, for example, it is Tuesday the 13th that is considered unlucky. In Italy, it is Friday the 17th.

The fear of Friday the 13th is called paraskavedekatriaphobia,[1], a word derived from the concatenation of the Greek words Paraskeví ( ±Á±ĂşµĹ®) (meaning Friday), and dekatreís (´µş±ÄÁµŻÂ) (meaning thirteen), attached to phobía (Ćż˛Ż±) (meaning fear). The term is a specialized form of triskaidekaphobia, a simple phobia (fear) of the number thirteen appearing in any case.