It is simultaneously a lucky and unlucky number. It is considered to be a bad omen because the Knights Templars were arrested on October 13, 1307. However it is also the number of reincarnation and everlasting life.
Witches' covens traditionally have thirteen members. The number of the cult (there are 13 members in a covens and satanic cults (12 regular members and one high priest). This is a mirroring of 12 jurors+1 judge=13. It is also believed that there are 13 high occult families that run the world
Friday, subjected to Venus, is widely held to be unlucky and a day when evil influences are at work - especially if it happens to be the thirteen day of the month. A day when anything that can go wrong will go wrong. The day that Eve offered the apple to Adam in the Garden of Eden, Friday is favoured for the holding of witches' covens and was formerly the customary day for hangings.
On a Friday the 13th "the lady in white", better known as "Our Lady of Fatima" chose to manifest herself and to transmit a message of luck and happiness.
Accents are more frequent on Fridays (though visits to the doctors are not advised), and clothes made on this day will not fit. Projects or trips begun on Friday will not prosper and any person who laughs on that day, says one proverb, will cry on Sunday. Many people add that Friday is also inauspicious day for moving house or for weddings, and formerly in some parts if the British Isles those who courted their lovers on a Friday were hounded by friends and neighbours banging noisily on pans and kettles. According to one old Shropshire superstition, news received on a Friday makes a physical impression upon the hearer in the form of a new wrinkle for every tiding. Children born on Friday will prove unlucky, but will enjoy the gift of second sight and healing powers ( I was born on a Friday) Many claim that the weather on Friday will be repeated on the following Sunday. New undertakings of many other events of a personal nature, will also be postponed.
In reality, the prejudice against the number thirteen is of obscure origins, as evidence exists of it in Roman civilization long before Christ and the Last Supper, which is none the less usually cited as the source of superstition. Perhaps significantly, the number thirteen was to the ancient Egyptians the last step of the ladder via which the soul reached eternity, though other authorities have suggested Hindu origins. http://witchesofthecraft.com/tag/friday-13th/
*The Cassell Dictionary of Superstitions Author David Pickering