Friday the 13th is an unlucky day according to many people. That's irrational as are other faith-based belief systems.
Many bad things happened in the Bible on Friday. Adam and Eve got kicked out on a Friday, Noah's flood happened on a Friday, the Tower thing fell on a Friday. Does it say that in the Bible? Jesus was crucified on a Friday. We know this because they had to get the crucifixion finished before the Sabbath started. Also Friday was named after the Norse goddess Freya, who was the goddess of love. Well the Pope didn't want people to get mushy on a fictional god (although she was just as fictional as his god) and said that Friday was the witch's sabbath. Friday wasn't a day to be messed with.
Beside the fact that Friday is the 13th day of the week (starting from Sunday and going back again). Many other religions didn't like 13. In Norse mythology there was a party for the gods, but when the 13th god showed up, things went wrong.
Also, the Last Supper had 13 people at it.
The ancient Egyptians believed that life was 13 steps, and on the 13th step you die. However, they thought the better type of afterlife was cool.
Astrophysicist Neil deGrasse Tyson has pointed out that most hotels in New York don't have a 13th floor. Specifically, the elevator control keys omit the number 13 - the button 12 is followed by the button 14. He claims that superstitious beliefs like these are bad for the United States. Polls suggest many Americans would be uncomfortable about having a room on the 13th floor. 
Friday 13th scares people
Fear of Friday the 13th has made people lose money. Also accidents happen on Friday the 13th more than on other days because superstitious people are scared and make mistakes. And they like telling these stories to reinforce their beliefs.
If people believe in the superstition of Friday the 13th then they believe they are in greater danger on that day. As a result they may be more anxious and distracted and this could lead to accidents. It becomes a self fulfilling prophecy. It is like telling someone they are cursed. If they believe they are then they will worry, their blood pressure will go up and they put themselves at risk. (Dr Caroline Watt of Edinburgh University) 
Good things that happened on Friday the 13th
Here's what the Committee for Skeptical Enquiry has to say.
At the end of the day, skeptics hope to at least set the public record straight for the maligned number 13. After all, 13 American colonies united to make a very successful country. And looked at objectively, the 13th comes out looking pretty good as far as days of the month go. Some very successful people-including Harrison Ford, Alfred Hitchcock and Stevie Wonder-were born on the 13th, and Friday, October 13, 2000, marks the third anniversary of British fighter pilot Andy Green's new land speed record in the Thrust SuperSonic vehicle, which broke the sound barrier in Nevada's Black Rock Desert.
- CSICOP's Friday the 13th Superstition Bash Every Friday 13th the Committee for Skeptical Enquiry hold cool parties when they do stuff like crossing the path of a black cat to show they're not scared of superstition.
- What is the origin of the Friday the 13th superstition?
- How Friday the 13th Works