The number 13 is associated with bad luck in some countries, and even has a specifically recognized phobia, Triskaidekaphobia, a word which was coined in 1911. Friday the 13th has been considered an unlucky day since the 1800s, as a combination between an unlucky day, Friday, and the number 13.
Triskaidekaphobia (from Greek tris meaning "3," kai meaning "and," and deka meaning "10") is fear of the number 13; it is a superstition and related to a specific fear of Friday the 13th, called paraskevidekatriaphobia or friggatriskaidekaphobia.
Triskaidekaphobia may have also affected the Vikings—it is believed that Loki in the Norse pantheon was the 13th god. More specifically, Loki was believed to have engineered the murder of Baldr, and was the 13th guest to arrive at the funeral. This is perhaps related to the superstition that if thirteen people gather, one of them will die in the following year. Another Norse tradition involves the myth of Norna-Gest: when the uninvited norns showed up at his birthday celebration—thus increasing the number of guests from ten to thirteen— the norns cursed the infant by magically binding his lifespan to that of a mystic candle they presented to him.
Ancient Persians believed the twelve constellations in the Zodiac controlled the months of the year, and each ruled the earth for a thousand years at the end of which the sky and earth collapsed in chaos. Therefore, the thirteenth is identified with chaos and the reason Persians leave their houses to avoid bad luck on the thirteenth day of the Persian Calendar, a tradition called Sizdah Bedar.
There is a common myth that the earliest reference to thirteen being unlucky or evil is from the Babylonian Code of Hammurabi (circa 1780 B.C.E.), where the thirteenth law is omitted.
* There were thirteen participants at the Last Supper. Tradition states that Judas Iscariot , the disciple who betrayed Jesus, was the 13th to sit at the table.
* Thirteen was once associated with the Epiphany by Christians, as it is said the child Jesus received the Magi on his thirteenth day of life.
* There are 13 Apostles in Christianity; the 12 Disciples plus St Paul, formerly Saul of Tarsus. In addition to being the 13th Apostle, he is also known as "The Apostle" by the Church Fathers.
* In Judaism, 13 signifies the age at which a boy matures and becomes a Bar Mitzvah (Age of 12 for Girls, or Bat Mitzvah).
* The number of principles of Jewish faith according to Maimonides
* According to the Torah, God has 13 Attributes of Mercy
* The number of circles, or "nodes", that make up Metatron's Cube in Kaballistic teachings.
Some Christian traditions have it that at the Last Supper, Judas, the disciple who betrayed Jesus, was the 13th to sit at the table. However, the number 13 is not uniformly bad in the Judeo-Christian tradition. For example, the 13 attributes of God (also called the thirteen attributes of mercy) are enumerated in the Torah (Exodus 34: 6-7). Some modern Christian churches also use 13 attributes of God in sermons.
On thirteenth day of death a feast is organized. It is believed to be organized for the peace of the departed soul.
More on the # 13 folklore/beliefs/mysteries 13: A Secret Number of Sacred Power
PERSONALLY I CONSIDER IT TO BE A LUCKY NUMBER.