Thirteen Being Unlucky:

The fear of the number thirteen (13) is so pervasive that it even has it's own fancy Greek term: triskaidekaphobia. The belief that thirteen brings bad luck is an extremely pervasive belief throughout many societies, and is strong enough that many major hotels and high rises traditionally either build only twelve floors, or, if they want to go higher, skip labeling the 13th floor entirely! Many people refuse to stay on the 13th floor, or in room 13. People stay home from work, for fear of something bad happening. Most airports don't have a thirteenth gate. And in Topeka, Kansas, where the zip code starts with 666- (really, it does! I know because I live there), they skip from 66612 to 66614 – which highly stinks because it would be highly
notorious to be able to boast having 66613 as my zip code.

There are many theories as to why this belief is held. One is that Judas, known as "the Betrayer of Jesus", was the 13th member present at the Last Supper.
Not all ancient cultures held the number in fear. The Chinese and the Egyptians thought of it as lucky.
"To the ancient Egyptians, we are told, life was a quest for spiritual ascension which unfolded in stages — 12 in this life and a 13th beyond, thought to be the eternal afterlife. The number 13 therefore symbolized death — not in terms of dust and decay, but as a glorious and desirable transformation. Though Egyptian civilization perished, the death symbolism they conferred on the number 13 survived, only to be corrupted by later cultures who associated it with a fear of death instead of a reverence for the afterlife." ("Why Friday the Thirteenth is Unlucky", p. 2,
Urban Legends )

In the Code of Hammurabi, an early law code dating from ancient Babylon, the laws are numbered and skip from 12 to 14. It is not clear why the Babylonians
considered 13 to be extremely unlucky. Matt Rhodes
(MRhodes@FARS.IDINC.COM) offers one explanation: "One of my English professors from college (Mythology class) told me that the earliest documented
example of the number thirteen as something bad came from the Song of Ishtar, an ancient Babylonian epic poem. The thirteenth line contains the name of the
Goddess of the Dead (which is never a good thing)."
Unlucky Friday the 13th.

This one is closely related to the previous superstition. Paraskevidekatriaphobia: is the official term for the fear of Friday the Thirteenth. Jesus was said to have
been crucified on Friday and the number of guests at the party of the Last Supper was 13, with the 13th guest being Judas, the traitor. There is also the tradition that roots of this belief stem from when the order came to rout out the Knights Templar on Friday the 13th. Many were rounded up and killed. "...It was a well coordinated raid that took place on Friday the 13th. The action was so swift, brutal and efficient that the day has lived on in infamy ever since," writes Richard Douek (richard_douek@mvbms.com).

"On October 13, 1307, a day so infamous that Friday the 13th would become a synonym for ill fortune, officers of King Philip IV [Philip the Fair] of France
carried out mass arrests in a well-coordinated dawn raid that left several thousand Templars — knights, sergeants, priests, and serving brethren — in chains, charged with heresy, blasphemy, various obscenities, and homosexual practices. None of these charges was ever proven, even in France — and the Order was found innocent elsewhere — but in the seven years following the arrests, hundreds of Templars suffered excruciating tortures intended to force "confessions". More than a hundred died under torture or were executed by
burning at the stake." (Katharine Kurtz, Tales of the Knights Templar, Warner
Books, 1995).