Friday the Thirteenth, the most dreaded date of the calendar. It has darkened our lives since the 19th century: the first known superstitious reference to the date is in 1869, when Gioachino Rossini died on Friday 13th November, the very date he had particularly believed to be so unlucky.
Fear of this day is called paraskevidekatriaphobia: paraskevi being Greek for 'Friday,' and dekatria meaning 'thirteen.' It is estimated that around $900 million USD are lost in business on this day, as people are reluctant to go out, drive, trade or any other potentially risky things. Statistics do actually show a slight increase in road accidents on Friday the 13th, compared to any other Friday, but probably this can be explained by the simple fact that what you expect to happen, often does.
The reasons for this superstition are complex. Friday has always been considered an unlucky day: this is referred to in Chaucer's Canterbury Tales written in the 14th century. This possibly stems from the crucifixion of Jesus, which took place on a Friday. Thirteen is the traditional unlucky number. Again this may link to the crucifixion: there were thirteen people at the Last Supper. Thirteen is a discordant number: being prime, it is divisible by no numbers except itself and one, whereas twelve is the traditional number of harmony and completeness. There were twelve apostles, twelve knights of the round table, twelve constellations of the zodiac, and many others. Friday and thirteen combined is therefore a double whammy of bad luck.
Another reason, popularised by Dan Brown's The Da Vinci Code, is the Knights Templar, the organisation linked to so many conspiracy theories which were rounded up en masse and imprisoned or killed on Friday 13th October 1307. This may have led to a belief that this date was cursed.
Whether you believe in it or not, I hope the day passes well for you.