Friday the 13th is one of those days that makes people feel eerie, though they may not really know why.

Like walking under a ladder or breaking a mirror, everyone knows Friday the 13th is renowned for bringing bad luck, but there's a lot more to know about this spooky day:

1. This year there are three Friday the 13ths, 13 weeks apart. The fact that there are three Friday the 13ths this year is eerie enough (most years there are two; last year there was only one), but the fact that they are 13 weeks apart is a major anomaly. It hasn't happened since 1984.

2. In 1993, a study in a British Medical Journal ran a study called Is Friday the 13th Bad for Your Health? that found that though less people drove on Friday the 13th than on Friday the 6th over a period of years, there was a higher number of traffic accidents on Friday the 13ths. Friday 13th is unlucky for some, the study concluded, according to About.com. The risk of hospital admission as a result of a transport accident may be increased by as much as 52 percent. Staying at home is recommended.

3. There appears to be no recorded evidence of a fear or superstition regarding Friday 13th before the 1800s. The earliest documented English reference to the day being unlucky was in an 1869 biography by Henry Sutherland Edwards.

4. Friday the 13th is such a freaky day for some people that a termed for the intense fear of the day: friggatriskaidekaphobia or paraskevidekatriaphobia. Try saying those terms 13 times fast.

5. According to the Bible, thirteen people are said to have attended the Last Supper before Jesus died on a cross on a Friday. The scene is related in John 13:21.

6. The Church of Satan rings in Friday the 13th as a day when superstitions grip the pious: Celebrate Friday The 13th! Today is 'Friday the 13th,' and we Satanists enjoy the consternation a simple number can conjure amongst the superstitious, the church posted on its official website Friday morning. So, on this dreaded day, while the credulous cower we sally forth to indulge with gusto. Have a truly splendid day today and an even more enriching night, dear aficionados of the outré!

7. Rapper Tupac Shakur died on Friday, Sept. 13, 1996, after having been shot seven days earlier.

8. President Franklin D. Roosevelt was so superstitious that he refused to ride a train on Friday the 13th, Thomas Fernsler, a University of Delaware mathematician so versed in lore about the number that he is known as Dr. 13, told USA Today.

9. The word coven has long been used to describe a gathering of 13 witches.

10. Many sources say that fear of Friday the 13th is the #1 superstition held by Americans.

11. A Norse myth promises dire consequences for people who choose to eat in groups of 13 diners, according to USA Today.

12. Most people know that many buildings have no 13th floor, but some cities go a step further by not even having a 13th Street or 13th Avenue.

13. The Knights Templar, of medieval fame, were imprisoned on Friday the 13th, according to USA Today.