why 13 is a mystery number?  

13 is deemed to be unlucky. That is why even buildings don't have 13th floor. But this assumption has no basis at all. I think the origins of this was Jesus Christ's 13 apostle. His 13th apostle who was Judas Escariot betrayed and give him away to the authority at that time and thus making possible for the crucifixion of Jesus Christ.

I guess it goes back to ancient times. do some researching. like on wikipedia

It's tagged in the Philippines as an unlucky number. If it gets to be on a friday in a month then people tend to work cautiously cause they say accidents are prone.. And it was indeed used in a movie with freddie in the Friday the 13th,

according to china prophecy, 1 plus 3 equals 4. 4 means death for them. so everything had ended with 4 just like 22 usually is avoid too. its a matter of your believe. if you believe it to happen, then it will.

We can't say 13 is a bad number besides I was born during friday the 13 and I'm proud of it. I look 13 in other view and I can say is its just only a number like other. Nowadays many people now are using the number 13 for unknown reasons Hmmmmm.

I do not know why number 13 has became an unlucky number. Perhaps because it is a prime number that cannot be divided by any other numbers, it stands alone as something mystique and in solitude. Besides it could be due to the number 3. More often we see items were sold in pack of threes instead of pairs, such as three apples for $2.00, three pens in a pack, three flavors of ice cream in a bundle and so on. Therefore it is neither too little nor too much -- it just arouse you to fix your sight and decision on the pack of three items, so that you won't consider other options. Likewise, the number 13 may cause one to solely fix his mind on linking it to something anomalous, unnatural, supernatural or bizarre. Having said that, I have not met anything unfortunate or been able to associate anything for that matter to number 13. My Fridays the 13th were spent like any other days, some with joy and excitement. My 13th year of age was a memorable time, when I started a new phase of life in secondary school as a teenager. I don't mind buying anything associated with number 13. Now I remember I watched a movie called "The Thirteenth Floor." Therefore I tend to believe it is how our inner life responds to the environment. If our lives were filled with hopelessness, helplessness then the environment will take its effect on us, be it it has anything to do with number 13 or not.

I have myself not discovered anything about the number that stands out in my life. But you did make me curious to read about what is up with the number 13? Well is seems that Triskaidekaphobia has many possible myths behind it. One of the earliest references seem to come from Bablylonian Code of Hammurabi. It was said that the 13th law was omitted. In fact though the original Code of Hammurabi has no numeration. Then there are some Christians who believe that during the last supper Judas was the 13th person to sit at the table. Norse legend says that twelve gods were summoned to a banquet at Valhalla. Loki the god of evil and turmoil crashed the party and Impaled Balder with a spear of mistletoe. Perhaps that is where the saying came from that if 13 people gather together one will die the following year?
The supposed 13th superstition exists all through Europe. The French used to have socialites known as quatorziems who would make themselves available to be the 14th guest at a dinner gathering hoping to keep them from unlucky fate. Also in France they say that it is impossible to find a house or a room in a hotel with the number 13. British tradition associates Friday with the number 13. Friday the popular day for public hangings is said that the steps leading to the noose were 13. Hmm. Also it has been said that witches gathered in groups of 12 in ancient Rome and the 13th was the devil. Then comes the oh if you have 13 letters to your name you will have the devils luck. A couple of examples are Charles Manson, Jack the Ripper. I told this to my husband and he started counting his , dang they came to 13. LOL
Ancient Persians believed that the twelve zodiac constellations controlled the months in the year each ruling for a thousand years. At the end of this time the sky and earth would collapse into chaos. Giving the number to symbolic to chaos.
In the late 1800s a group of influential people that would eventually include 5 presidents formed a Thirteen Club. Their first meeting held on Friday the 13th, shocking, there were 13 people who joined for dinner etc. On Friday the 13th in October of 1307 Philip V of France ordered the Knights Templar arrested. Maybe our founding fathers found it important to preserve this for the sake of Freemasons. Even though there are many people that have moved on from superstitions developers are still weary of having a 13th floor designated as such. It seems that it is a hard floor to sale or lease etc. Which can make it hard because when there is a rescue involved and people say hey I am on the 14th floor. They are in fact actually on the 13th and can cause precious time.
I would also like to point out that other countries also have numbers that they hold as unlucky. Italy has there own myths for the number 17. In Japan, China, Korea and some other Asian countries 4 is said to be unlucky. Ok I am done rattling just wanted to share what I found and thought was interesting.

I think it's just a superstition. If the 13th of the month falls on a friday, it's commonly known as the black friday which the westerners believed to be an unlucky day. Therefore, the number became a unlucky number for them. For Chinese, they also believe in the unlucky number 4, since it's pronounced the same way as death in chinese. Therefore, most Chinese would avoid places with the number 4. 4th floor or 4 block, etc. I personally don't really believe in all those, it doesn't have any scientific basis.

they say 13 is a bad luck number, because mostly the incident happened on that day. maybe it is just a coincidence but successive happening is not good. maybe it has a batter explanation on that matter, and explanation we have to find out. it has been traditional beliefs also that we are following. no harm in following, so we better follow to avoid bad luck. that is what i think. have a nice day!

the number 13 is deemed as an unlucky number and fro what i read it stemmed from the unlucky day, friday the 13th. friday the 13th came from the time when all of the templars were all rounded up and all tortured or killed. but i think there are other reasons on why 13 became an unlucky number in various cultures.