Happy Friday 13th. There are many stories about why it's considered unlucky. But the origins of friggatriskaidekaphobia are generally uncertain.
The number 13 has been thought unlucky for at least a few hundred years. Twelve is considered a good number in many religions--the number of apostles, the months of the years and signs of the zodiac, etc. Thirteen is irregular, and apparently, therefore, suspicious. There were thirteen people at the Last Supper, and that didn't turn out well. Friday's bad rep may be due to that's the day Christians commemorate the crucifixion of Jesus.
The fear of Friday the 13th seems to be a fairly modern phenomenon, not much older than a century. Whenever people bring it up, I say "It can be the 1st, the 13th, the 31st...I don't care. What counts is it's Friday."